Solution: I downgraded websocket gwt library version in html module to
api 'com.github.czyzby:gdx-websocket-gwt:1.8.1.9.4' api 'com.github.czyzby:gdx-websocket-gwt:1.8.1.9.4:sources' api 'com.github.czyzby:gdx-websocket-gwt:1.8.1.9.4' api 'com.github.czyzby:gdx-websocket-gwt:1.8.1.9.4:sources' AndAnd now it sees the source code for all websocket classes.