J2CL attempts to solve a much different problem than GWT. It does not try to replace js or js developers.

J2CL is designed for cross platform code reuse. For example, the code powering Google Docs front end needs to be written for web, Android and iOS. J2CL does the transpilation of all document manipulation and rendering logic from Java -> JS (and accomplishes this with unprecedented js code size). The UI is still written in JS.

Java -> Android

Java -> Backend Server

Java -> J2objc -> iOS

Java -> J2CL -> JS/web.