The gradle file is setup so that webpack is always run before bootRun. There are also utility watchJS script files that can be run on *nix/Windows which will watch your JS files and recreate a bundle. You'll need to refresh your page to see the changes though but you will not need to restart your Spring Boot app.
For a better step by step build up on this project, check out the blog post associated with this project. If you're a Java developer new to the JS world, then it's worth a read as I go over npm and package.json and how they are used.
The watch scripts can be used after initializing the project. Type
./gradelw bootRun (or run via gralde task in IDE) then you can use the watch script. Running
gralelw bootRun or
gradlew build will initialize the node_modules directory.