>>174
If they GPL their fork, it's perfectly within their rights. Though if they choose to charge for it while you have the same software e.g. on git for free, people will probably prefer your free-as-in-free-beer version.
If they don't GPL it then you have legal grounds to get them to cease and decist distributing it until they comply with the terms of the GPL. Also, what >>175-san said. There are not many free as in freedom apps in any of the major mobile app stores to begin with because those companies prefer to have both developers and users under the control of their proprietary systems (with regards to Google, technically, Android/Linux is free software, but it is almost always distributed with the Google Play app which is not freedom respecting.)