I use GWT at work daily. Though we don't use GWT RPC at all and just have normal http endpoints returning JSON on the server, so I wouldn't describe it as tight

I use GWT at work daily. Though we don't use GWT RPC at all and just have normal http endpoints returning JSON on the server, so I wouldn't describe it as tightly coupled to the RPC system. That said, I wouldn't recommend it for new projects.