diff --git a/src/v1/secret_manager_service_client.ts b/src/v1/secret_manager_service_client.ts
index dc68d49..47ebda2 100644
--- a/src/v1/secret_manager_service_client.ts
+++ b/src/v1/secret_manager_service_client.ts
@@ -17,8 +17,8 @@
 // ** All changes to this file may be overwritten. **
 
 /* global window */
-import * as gax from 'google-gax';
-import {
+import type * as gax from 'google-gax';
+import type {
   Callback,
   CallOptions,
   Descriptors,
@@ -26,7 +26,6 @@ import {
   PaginationCallback,
   GaxCall,
 } from 'google-gax';
-
 import {Transform} from 'stream';
 import * as protos from '../../protos/protos';
 import jsonProtos = require('../../protos/protos.json');
@@ -36,7 +35,6 @@ import jsonProtos = require('../../protos/protos.json');
  * This file defines retry strategy and timeouts for all API methods in this library.
  */
 import * as gapicConfig from './secret_manager_service_client_config.json';
-
 const version = require('../../../package.json').version;
 
 /**
@@ -102,8 +100,18 @@ export class SecretManagerServiceClient {
    *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
    *     For more information, please check the
    *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
+   * @param {gax} [gaxInstance]: loaded instance of `google-gax`. Useful if you
+   *     need to avoid loading the default gRPC version and want to use the fallback
+   *     HTTP implementation. Load only fallback version and pass it to the constructor:
+   *     ```
+   *     const gax = require('google-gax/build/src/fallback'); // avoids loading google-gax with gRPC
+   *     const client = new SecretManagerServiceClient({fallback: 'rest'}, gax);
+   *     ```
    */
-  constructor(opts?: ClientOptions) {
+  constructor(
+    opts?: ClientOptions,
+    gaxInstance?: typeof gax | typeof gax.fallback
+  ) {
     // Ensure that options include all the required fields.
     const staticMembers = this.constructor as typeof SecretManagerServiceClient;
     const servicePath =
@@ -123,8 +131,13 @@ export class SecretManagerServiceClient {
       opts['scopes'] = staticMembers.scopes;
     }
 
+    // Load google-gax module synchronously if needed
+    if (!gaxInstance) {
+      gaxInstance = require('google-gax') as typeof gax;
+    }
+
     // Choose either gRPC or proto-over-HTTP implementation of google-gax.
-    this._gaxModule = opts.fallback ? gax.fallback : gax;
+    this._gaxModule = opts.fallback ? gaxInstance.fallback : gaxInstance;
 
     // Create a `gaxGrpc` object, with any grpc-specific options sent to the client.
     this._gaxGrpc = new this._gaxModule.GrpcClient(opts);
@@ -212,7 +225,7 @@ export class SecretManagerServiceClient {
     this.innerApiCalls = {};
 
     // Add a warn function to the client constructor so it can be easily tested.
-    this.warn = gax.warn;
+    this.warn = this._gaxModule.warn;
   }
 
   /**
@@ -441,7 +454,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     this.initialize();
@@ -536,7 +549,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     this.initialize();
@@ -621,7 +634,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -714,7 +727,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         'secret.name': request.secret!.name || '',
       });
     this.initialize();
@@ -810,7 +823,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -908,7 +921,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -1012,7 +1025,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -1117,7 +1130,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -1222,7 +1235,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -1328,7 +1341,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         name: request.name || '',
       });
     this.initialize();
@@ -1427,7 +1440,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         resource: request.resource || '',
       });
     this.initialize();
@@ -1515,7 +1528,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         resource: request.resource || '',
       });
     this.initialize();
@@ -1610,7 +1623,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         resource: request.resource || '',
       });
     this.initialize();
@@ -1719,7 +1732,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     this.initialize();
@@ -1767,7 +1780,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     const defaultCallSettings = this._defaults['listSecrets'];
@@ -1824,7 +1837,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     const defaultCallSettings = this._defaults['listSecrets'];
@@ -1940,7 +1953,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     this.initialize();
@@ -1989,7 +2002,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     const defaultCallSettings = this._defaults['listSecretVersions'];
@@ -2047,7 +2060,7 @@ export class SecretManagerServiceClient {
     options.otherArgs = options.otherArgs || {};
     options.otherArgs.headers = options.otherArgs.headers || {};
     options.otherArgs.headers['x-goog-request-params'] =
-      gax.routingHeader.fromParams({
+      this._gaxModule.routingHeader.fromParams({
         parent: request.parent || '',
       });
     const defaultCallSettings = this._defaults['listSecretVersions'];