A very brief look over gjtorikian's repo indicates that there are copyright headers and licence/copying information available, so there shouldn't really be any room for confusion once things are spelled out.
I don't recall whether we've put licence details on non-code contributions (I don't recall putting anything on the stuff I've added, and I notice that my orange ball features in their screenshots), but I guess we should probably make sure that that's considered as well.
I haven't had time to really look into this, but if somebody's willing (if that somebody ends up being me, it's going to be a while before I can squeeze time in to give this the attention it deserves), I think contact should probably be made (as tones says, a diplomatic solution is likely to result in more positive outcomes - open hands before warning shots), and then we can look at what context of the situation is and triple check that they're not breach of licence terms (I don't have an android device, so I can't check that they're not giving a link to a repo somewhere in a readme).
It'd be nice to have active commits back to gjtorikian's repo, and if they wanted to be active here, then that'd be awesome, but I think that having them publish their modifications to the source of our GPL app (I did spot some Apache licenced stuff in gjtorikian's repo as well) is as much as we can fairly expect, and is as much as is required by our licence terms.
Edit: One possible option for making contact could be to post a comment on their bloggy thing asking them to join the discussion here. I'm not certain how confrontational that might seem though (it could be a fairly public place to be broaching what could end up being a fairly sticky issue).