I was able to get the tablet setup using GAIA and the GPS trailmasters gpx files, and used them both this past weekend up north in NH (I'll make a separate post regarding the trip). The only hiccup I ran into during the initial download/install was the gpx file sizes.
Gaia only allows files to have 1000 features (at least in the free version) in order to be uploaded. The guidance from Gaia is to split large files into multiple smaller files, then simply upload multiple smaller files to have all of the info. Gaia also recommends a couple gpx editor apps, but I couldn't get the non-Mac app to work. I ended up using gpx.studio after a quick google search. Using gpx.studio was rather straight forward, you upload your gpx file to be edited, you then see the features displayed on a map and also a profile view at the bottom of the screen showing feature 1 as start and the last feature as stop. In the profile view you can drag the start and stop to different locations along the profile, essentially cropping the files down below 1000 features each and then save them off for upload into gaia. I ended up splitting NH into 2 files (1500 something features originally) and then uploaded them both to the app. I haven't dealt with the Maine files yet, but a quick look showed 6000+ features, so at least 6 smaller files will need to be created. If you pay attention to the map as you adjust the profile toggles, you can see where the features are, and break the files into regions if you choose. For NH I just did a north and a south file.
The GPS trailmasters files contain both trails and waypoint such as gas, food, picnic areas, and scenic overlooks. For the most part the files were pretty accurate, but there were places where the signs on the trails or the trails themselves did not match, so don't take it as gospel. I was able to track our rides each day and save them for future viewing. I was also able to add additional waypoints for difficult trail sections, parking spots near streams, etc. Overall I'm happy with the GPS trailmasters purchase and would recommend the maps to others if they are within their coverage area (basically New England and Penn).
If you have any questions feel free to ask.