What do you mean by flow? You will have to be a bit more specific here.
As a developer, you write pure Java code. The choice of classes is limited to white listed ones which can be emulated into JS.
Then the GWT "compiler" converts all this java code into JS. That is it, on a very high level, as far as GWT is concerned. Essentially GWT is a front end framework.
For communicating with the server side (so that you can get/set data, and update your UI), one typically uses the RPC mechanism.