You can stop gaining XP? How's that done?
1: Place a new folder, named "Mods" into C:\Program Files (x86)\GalaxyClient\Games\The Witcher 3 Wild Hunt
2 Go to
C:\Program Files (x86)\GalaxyClient\Games\The Witcher 3 Wild Hunt\content\content0\scripts\game\gameplay\leveling and copy the file "levelManager.ws" (we need to modify this file but two of those have to exist, so don't delete the original, just copy it and save it to the desktop).
3 Open the levelManager.ws you saved on the desktop (with the usual NotePad). In this file you'll see three times the number "70". Just replace 70 with the level of your character and that will be the new level cap. Save this file.
4 Go back to the Mods folder you created in
C:\Program Files (x86)\GalaxyClient\Games\The Witcher 3 Wild Hunt\Mods
5 Now you have to create a few subfolders within the Mods folder, so that in the end it should be:
C:\Program Files (x86)\GalaxyClient\Games\The Witcher 3 Wild Hunt\Mods\modMaxLevel\content\scripts\game\gameplay\leveling
and within the last folder "leveling" you can put that text file we edited before, levelManager.ws (we're mimicking the original levelManager location)
That's it, just launch the game.
If you want to remove the level cap you either erase the file (or the whole Mods folder) or edit again the file and select an higher level cap.
P.S. Just editing the original file doesn't work as The Witcher 3 loads many parameters from cached files that are impossible to find,
so that whole mods folder thing is unavoidable and is the only thing that forces that levelManager.ws to be read at every startup.
And this is why I love PcGaming, if you want to fix something, you can.