Hi...I recently discovered that with account link, I could redeem TL games from steam onto GOG.
I'd prefer to play it on GOG, but are steam saves compatbile with GOG?

Yes, just copy your old save folder in the GOG installation of the game.

The Steam saves are stored in C:\Users\(User Name)\Documents\My games\Runic Games\Torchlight 2\save\(long number)

GOG saves are in C:\Users\(User Name)\Documents\My games\Runic Games\Torchlight 2\save\

Just go into the (long number) folder, copy the files, then go back to save and past them.