Hello guys,
I bought CP 2077 on GOG.com and this will be the first time I will use Wemod on a GOG game.
It always worked on EGS and Steam. But on GOG you have multiple versions of a game that you can download. There is a GOG Galaxy version and several offline versions. Do you guys know what version of the game is supported by the wemod trainers on gog games? Maybe someone knows how it was in Witcher 3?
Welcome to the community. 
As long as the game came from GOG legitimately, the trainer (when it is available) should work perfectly fine, I believe.
GOG Galaxy is just a launcher. All it does is it launches the game that is installed on your PC.
If you have an issue with launching the game through the trainer, simply launching the game first and then alt+tab to WeMod to press Play will usually resolve it.