If you have a paid MapBox.com subscription (Standard - $49/month) , you can use Mapbox.com to import many kinds of maps/data into Gaia GPS.
1) Start out by importing a geotiff or other georeferenced images such as .geojson, .gpx, .csv, .kml, .tif, or mbtiles into mapbox.com.
2) After the file has finished processing go to the data page and click the file to open a menu. Click preview.
3) On the next page, right click the image and copy the image URL. The URL should look something like this:
https://a.tiles.mapbox.com/v4/trailbehind.to4m42t9/8/133/89.jpg?access_token=pk.eyJ1IjoidHJhaWxiZWhpbmQiLCJhIjoiSElESjdodyJ9.Kx2Uq40YykPyf19UKVMNLA
4) Import to gaiagps.com
To upload the TMS URL directly to your gaiagps.com account, follow the instructions here.
or
Create a TileJSON file (iOS only)
For help creating a TileJSON, view the instructions here.
When putting the URL into the TileJSON file, replace the highlighted values (8/133/89) with 'xyz' variables as show below, with the smallest value becoming z.
Example:
https://a.tiles.mapbox.com/v4/trailbehind.to4m42t9/{z}/{x}/{y}.jpg?access_token=pk.eyJ1IjoidHJhaWxiZWhpbmQiLCJhIjoiSElESjdodyJ9.Kx2Uq40YykPyf19UKVMNLA
Once you have a correct JSON, you can import to Gaia GPS via iTunes.