ya, I know it isn't for debugging, but assertions, but since GWT refers to it as debugging even in their description, I sometimes refer to it that way.
The issue we had is that we are using a GWT gradle script that (unknown to me) did -ea by default. After a variety of checks, I realized that it was adding -ea (you have to do debug=false in the gradle script to not call -ea). So, our solution is not to call -ea, since we don't want it in production anyway.
However, it made me wonder if this was a known limitation with extgwt, or something we should investigate. In any case, our work around is not to use -ea.
I had tried pretty mode, but didn't provide me any useful information at the time. However, I may go back and try it again.