Johninadelaide67: I installed it via the client and I think that was the issue. I got the offline installer and now it runs on its own without the need for the client. I should have realised that ... Oh well .
Berzerk2k2: When you install it via client the shortcut Galaxy will create is configured in a way that it will always start Galaxy before the game will launch. You can change that by deleting the shortcut, going into the game folder and copy the "Launch game name" file to your Desktop. Then you have a normal shortcut again. Works for every game that is sold here on GOG.
THIS POST, however old, isn't only the RIGHT answer to the original question, it's a feature I didn't know was available until I read it here.
THIS POST sort of changes everything for me -- I always prefer Steam. always. GOG Galaxy I find slower, finnicky, always unable to process more than one download in queue. (I hate that - if trying to add another download the Windows confirmation window will always hang. shrug!) I also just don't like having more than one client open at a time, so going back and forth from Steam to GOG Galaxy (or Ubisoft Connect or EA Origin if I happen to be hating myself that day) just kind of sucks out loud.
however. HOWEVER. this?? I just tried it, it absolutely works. you click the icon, the game launches, no client at all. like it was 2001 or something. absolute game-changer (in the most literal sense possible, laff).
this one feature DOES make me favor GOG as a whole a lot more. a LOT more. this is awesome. thank you, sir.
-b23