I keep a local copy saved as a plain text file formatted for GOG forum for posts like these. I only work off of it and copy/paste from it replacing everything when I hit edit. It may no longer like some URL of yours. Not sure how close to the character limit you are, but may want try to make it noticeably shorter.
If you don't have a local copy yet make one. Then make a backup of that file. Try to edit the first post to contain just the first line, "At least with Linux and Windows ports." Then try with half the post, etc. until you find where it breaks.

All I wanted to do was delete the WINE section since I made it external (third post) because of the character/line limit, you are probably right with the links though, the bug appeared after I completed the links so that our glorious forum software automatically makes hyper-links out of it, it doesn't seem to like one of the links it created itself when I repost ist via edit.
I'll remove the hyper-links again for now.

Fixed posts and added
World of Padman
worldofpadman.net/en/

Added
Daikatana
https://www.gog.com/game/daikatana
https://bitbucket.org/daikatana13/daikatana
Quake: The Offering
https://www.gog.com/game/quake_the_offering
http://nquake.com/
Quake II: Quad Damage
https://www.gog.com/game/quake_ii_quad_damage
http://skuller.net/q2pro/

Added the formerly commercial, now free and open source:
Thread Marks
www.longbowgames.com/treadmarks

I bought it back then, in a box.

Megaglest maybe?
I would have said Widelands too, but it still seems to be under development with no stable 1.0 release.

Factorio surely?
Nice list though.

Yeah. Factorio should be on the list. The website has windows, mac, linux. I dunno the GOG build but the website version is drm free too. If rimworld were multiplayer, I would say to add that. If mods for MP cound, add Kerbal Space Program

Added

Flare
flarerpg.org/