Depends on your situation of course. If you want to use your existing backend with other clients written in other languages then GWT-RPC is a bad fit. While the wire format of GWT-RPC is documented, you still need to write the client code to generate it. Also GWT-RPC supports inheritance which other languages you want to use might not support. Other JS based RPC solutions as well as general purpose solutions like gRPC typically do not support inheritance.
GWT-RPC will already be annoying if you decide to have some portions of your app being written in a different framework since it is easier to find developers for that framework, e.g. angular, svelte, react, whatever. You would then need to define a JS api that calls into GWT code so these frameworks can talk to your GWT-RPC backend (or you need to provide new endpoints in your backend that do not talk GWT-RPC).