I have an ultrawide monitor in a two monitor setup. Only the primary changes resolution as soon as Galaxy launches, but it goes from 3440x1440 to sth like 640x480, basically from the highest to the lowest.

This happens every time I launch Galaxy.

Lately I've been playing Secret Files Tunguska, which does use a similar low resolution with a pillarbox, but when I launch the game without Galaxy running, it changes the rs only for the game, but then reverts it.

Not sure if that's relevant and don't see why Galaxy should be bothered by it, but if it helps...

Even worse, I set Secret Files to be windowed from the config utility in its installation folder, but Galaxy still tried to go for a full screen pillarbox situation so I ended up with this monstrosity.

The left side is the 640x480 that Galaxy somehow sets to the screen, yet the game itself is windowed in the right screen along with the Galaxy client.

It may be a long shot but did you double check to make sure the Galaxy executable isn't set to "Run in 640 x 480 screen resolution" in the Compatibility tab when you right-click the shortcut?

Holy smokes, that was it...this is the first time I see an executable with that option checked since '95.....why?!
I've had it installed for a while now and don't remember having this issue before. Did a recent update mess it up?

Wait, really? That was the only thing that came to mind and I assumed that it almost certainly wouldn't work hahaha. I'm glad it helped, though! I don't know why it'd do that, though, it'd definitely be worth reporting it as a bug I'd say!

In my case it was my own stupid fault. I was trying to get Sacred 2 to launch and changed the shortcut resolution to "Run in yadda yadda..." It didn't change anything for Sacred 2, which still freezes on a screen with yellow text and the game logo, but it DID cause GOG Galaxy to run in 640x480 on update and relaunch.

Thanks to JakobFel for the suggestion. As soon as I read it I knew what the problem was. Appreciate you, mate!