From https://docs.larian.game/Definitive_Edition:
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GM Story Scripting
Story scripting and GM add-on support in general (when playing the campaign) will be brought live on the first patch after release.
Story (Osiris) scripting in Game Master mode is now fully supported!

This means that all your add-on scripting will work in Game Master, the same way it does in Story mode.
As always, simply make sure in your Project Settings (CTRL + P) that your project has the correct game mode enabled as Target.
Your project only works for the modes it targets! In this case, the target would be GM.
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So this means that if you can do scripting you have much more power. Notice that it say "ill be brought live on the first patch after release". I'm not sure what that means 100%. But lets see what happens.

This ALSO means that the amazing scipting modders out there can not only fiddle with the story mode but also GM mode. Lets hope someone takes up that challenge and give people like me with no lust for scripting some cool stuff.