I bought this for my son and he's running it just fine on his 780 GTX except for random flashes of geometry and texture, very similar to this post and the visuals posted:
https://www.reddit.com/r/horizon/comments/idkxbh/horizon_pc_texture_flickering_nvidia/
and here
https://imgur.com/a/VelNzFL

Any solutions mention deleting PSOCache.bin but that's with the Steam install. With his GOG install, the file isn't there or anywhere on the PC. We tried forcing 16x AA and no AA in the Geforce Control Panel with no success. I don't want the flickering to cause any migraines, so any help would be super appreciated.

You might have to turn shadows down or adjust other settings lower than they currently are at.

That particular flashing with older Nvidia cards are typically a sign from what I can tell from personal experience, that the card is being over taxed, even if you don't notice a huge fps drop.

I would set it to lowest settings and see if that fixes it, and if it does, slowly increase individual settings to see if it affects anything.

You might also want to re install the redistributables, which should be an option from the Extras menu on Galaxy.

Another thing that may fix it is defragging the hard drive, or possibly making sure the card is adequately cooled. He might have to set a custom fan ramp to make sure of that. The 780 was a good card, but I imagine that this game really stresses it.

I haven't had an Nvidia card in some time, so I apologize I'm not more helpful. I hope one of these suggestions fixes it for him. I also suffer from migraines but mine are of the air pressure sort, even so I hope he can continue to play this.

Have a great day!
-Pal