fix(controllers) skip routes bound to excluded GWs

In single-Gateway mode, properly skip route parent references for other
Gateways when determining whether to include a route.

CHANGELOG.md

@@ -89,6 +89,9 @@ Adding a new version? You'll need three changes:
 - When managed Kong gateways are OSS edition, KIC will not apply licenses to
   the Kong gateway instances to avoid invalid configurations.
   [#5640](https://github.com/Kong/kubernetes-ingress-controller/pull/5640)
+- Fixed an issue where single-Gateway mode did not actually filter out routes
+  associated with other Gateways in the controller class.
+  [#5642](https://github.com/Kong/kubernetes-ingress-controller/pull/5642)
 
 ## [3.1.0]
 

internal/controllers/gateway/route_utils.go

@@ -151,9 +151,13 @@ func getSupportedGatewayForRoute[T gatewayapi.RouteT](ctx context.Context, logge
 		// If the flag `--gateway-to-reconcile` is set, KIC will only reconcile the specified gateway.
 		// https://github.com/Kong/kubernetes-ingress-controller/issues/5322
 		if gatewayToReconcile, ok := specifiedGW.Get(); ok {
-			namespace = gatewayToReconcile.Namespace
-			name = gatewayToReconcile.Name
-
+			parentNamespace := route.GetNamespace()
+			if parentRef.Namespace != nil {
+				parentNamespace = string(*parentRef.Namespace)
+			}
+			if !(parentNamespace == gatewayToReconcile.Namespace && string(parentRef.Name) == gatewayToReconcile.Name) {
+				continue
+			}
 		}
 
 		// pull the Gateway object from the cached client