I have played this game a fair few times over the years, but now I can't seem to get it to run at all.

I do have the De Vespe Conspiracy DLC, which I've heard may cause this with mods... so I uninstalled, deleted all the mods and... it's still happening.

Some source on steam recommended the Codec Tweaking Tool, but that wasn't it.

I can't figure out what's going wrong and I also can't figure out how to play the game without the DLC (I guess I could install it without GOG Galaxy).

Any ideas?

I've played through the game plus the DLC with no issues using the offline installers (I don't use Galaxy). It may be worth seeing if you can start the game from the game executable and bypass Galaxy completely. Although I'm not sure if this can be done without invoking Galaxy but may be worth a try?

I stumbled upon the right answer.

Open "Documents\Greedfall\graphicconfig.txt" and make sure TEXTUREQUALITY is equal to 0, 1 or 2. Sometimes the game sets it as an absurd value and refuses to run.

Haven't verified mod compatibility with the DLC but I'm pretty sure this particular bug is unrelated to the DLC.