I am not an expert on this, but I expect you could communicate with an X11 client using just TCP for many things.
But just to get back to the original motivation of @rohqhq, which, if I understand correctly from another topic, is to learn programming and Julia: I think that starting both programming and Julia with GUI programming (of any kind) is not the best idea. While it is feasible, this is not where Julia shines.
Finally, both the other topic and this one focus on things that are not of primary relevance: protection from reverse engineering (I guess?) and writing everything in Julia only. These are distractions at best when one is learning programming.