I've just spent 2 hours getting GMLink to work properly, and here's the exact process I followed:
First problem encountered: GameMaker LTS v2.2, everything worked fine, but sprite thumbnails wouldn't load properly for any sprite imported with GMLink. Open sprite OR assign sprite to object and place in room = sprite just doesn't show up until runtime. Only exception was "edit image", which showed the sprite image, and the above problem would be fixed if I actually edited the image and saved changes. Tried disabling the thumbnails option in preferences, but that option doesn't exist in LTS v2.2. This wasn't a total deal-breaker, but it was inconvenient.
Tried: Switching to current version of GameMaker (NOT LTS) v2024.14.2.212. This required reimporting my project. Closed aseprite and GM, reimported and updated to latest version, everything appeared fine. Opened aseprite, switched GMLink config to new path, selected slice, pressed "update", and failed to find spr_gmlink_base, despite this sprite existing in my project. Tried reopen powershell, didn't work. Tried restarting aseprite and Gamemaker, didn't work. Tried deleting and recreating spr_gmlink_base, didn't work. Tried deleting old GMLink imported sprite from GameMaker entirely, then closing everything and reopening. Pressed "update" and bricked my project - error on reloading project + error on relaunching Gamemaker. Tried editing yyp file directly to delete keys, broke it even further.
Deleted project entirely, closed everything, reimported (again) to new GameMaker version. Deleted old GMLink sprite and spr_gmlink_base entirely. Then launched Aseprite, repeated steps above, now stuck on "can't find spr_gmlink_base". Tried everything I could think of - finally told GMLink to use the absolute path to my GM yyp project file. This worked - everything now working as intended. Relaunched both aseprite and GameMaker to test - all good. Hopefully this is helpful to someone!
PS thank you for the extension. If it keeps working, it saves a ton of clicks and time.