Add maps to Gaia GPS
You can add many of the digital maps you find on the web to Gaia GPS. Sometimes, it's as simple as pasting the map URL into Gaia GPS. Not every online map source will work, but if you have one in mind that uses the TMS (tile map service) protocol, you're in luck.
Find the URL
Gaia GPS has a several tutorials to assist you in working out the correct Tile URL. For example Adding a TMS Map Source
Adding off-road.guide map sources to Gaia GPS​
Open Gaia GPS in a web browser and click on the Layers link
Scroll down and click on the Add Customer Source link
The following dialog will pop up and you will need to enter the following information
Map Title: OSM France
Tile URL: https://a.tile.openstreetmap.fr/osmfr/{z}/{x}/{y}.png
Zoom Range: 6-18
And click on Save
The lauer will then appear in the Map Layer list
You can then treat the new layer as you would any other layer.
Download layer for offline use
Gaia GPS has a tutorial to assist in downloading maps for offline use. For example when you don't have any cellular coverage. See Download Maps for Offline Use