I’ve recently installed The Witcher directly from gog (not galaxy) and put it in a wine wrapper to play it on Catalina. I was hoping to add it to Galaxy but I only get the option to install, I can’t choose to link executable. Does anyone know a work around?

maybe you can try to add non-gog game via search (see gog's latest news for galaxy 2.0 - they explain how to find that feature in galaxy)

then add Witcher, in line where you link custom executable set line like you're launching it with wine.
wine path/to/witcher.exe

also if you have that game on your account already it might refuse to add another version, then just add different name. or name it Witcher Wine or similar

As I said before, I can’t link an executable, Galaxy doesn’t give me that option because it knows TW is on my account. Is there a way to change the link destination once a game is added?

I don't know about the Mac version of Galaxy, but with the Windows version i can add a game that i already own on GOG. Click on the "+" at the top of Galaxy, then "Add game manually". Now search for The Witcher and use the arrow keys to navigate to the entry you want. Then click "Mark as owned" and search for the game in your library. There should be a new symbol at the top left where you can choose what should be displayed. Choose the manually added game and link the executable. Look at the screenshots for further details.