Gaia GPS for Android update 6.5.12 adds the ability to import custom map sources directly into the app as an alternative to using the addmapsource page on gaiagps.com.
Create a TileJSON using a TMS map source by following the instructions in this Help Center article. Test out your TMS URL here: http://trailbehind.github.io/TilejsonTester/.
After creating the TileJSON file, Android users can save it to the internal storage or SD card. Navigate the filesystem from within Gaia GPS to import the file, then find the custom map source in the Layers menu when the import completes. Follow the instructions here for importing via the SD card.
You can also import TileJSON files in the iOS app, or import the TMS source directly to your account on gaiagps.com, then sync back to the app.
Read more about this release and others on the Gaia GPS updates page.