If you find a way to support their format, you might gain access to a huge repository of tabs. Maybe this might put your tool on Ubisoft's radar, so I suggest to thread lightly, but it would be awesome.
But you don't need anything nearly as complicated as Rocksmith files or CustomsForge to import songs into FATpick. FATpick already accepts Guitar Pro files, so there are thousands, maybe 10s of thousands of scores you can download from ultimate-guitar and other sites to import into FATpick.
Also, while obviously not as convenient, free software like TuxGuitar can convert other formats (MuseScore, MusicXML, etc.) into GuitarPro, so you can import pretty much any machine-readable score into FATpick (modulo proprietary formats like Rocksmith/CustomsForge, but you need to jump thru hoops to "extract" data in that format in the first place).
This is interesting. I suggest you add that somewhere in the home page.
I don't always reference "Guitar Pro" by name. Do you think that specific phrase would catch your eye or be more significant for you?
I.e. is "import Guitar Pro tabs" substantially more meaningful to you than, say, "import custom tabs"?
Full disclosure: right now FATpick will import gp3, gp4 and gp5 files. The "import song" button both enforces and mentions this, as does the user manual IIRC, but it's not something that's called out in any way.
If you have a gpx file (produced by Guitar Pro 6 or 7) then you'll need to convert it to gp5 to import into FATpick, at least for now. If you're using the GuitarPro app itself you can just save it that way. If you don't have the GuitarPro app TuxGuitar is free and handles this well (it works for editing GuitarPro files too).
Obviously it would be better to support .gpx directly, and for that matter things like MusicXML, too. But the GuitarPro format has to be more or less reverse-engineered, and gpx is unlike gp3, gp4 and gp5. Most files in the wild seem to be gp5 (or after that, gp3) anyway so that's where its at right now.
It seems like one could probably use some headless form of TuxGuitar on the back-end to convert gpx to gp5, but other than a cursory scan to see if that's a feature they already provide (it isn't) I haven't dug into it much more deeply than that.