Most map sources you use in other websites or programs can be imported into Gaia GPS, because they are "TMS" map sources. TMS map sources break maps into square tiles, and you can determine the pattern for the URLs for those tiles, and set up Gaia GPS to consume those tiles.
Most Common Method
The most common way to import a map source is to use this web form on gaiagps.com. It guides you through entering the URL for the source, naming it, and then displays tiles once it's working correctly.
You can also use external software such as MapBox, or create a small data file called a TileJSON file to import map sources. See the articles here for more information on these methods.
Manage/Use Map Sources on gaiagps.com
On gaiagps.com, add and manage your map sources by going to your custom map sources list. You can also get to that list by clicking the dropdown menu under your username (top right corner) and selecting "Map Sources."
Manage/Use Map Sources in the Gaia GPS App
The source will appear in the "More Layers" menu under "Imported."