I started up the game today after having not played for several days and discovered that you've automatically updated my installation to the Union version, changing all of the settings I had chosen (graphics mode) which let me play the game. Now the game just displays a mess of graphical gibberish, making it impossible to play. Not only that, but when I try to change the settings they keep being reverted back instead of using the settings I want.

THANK YOU EVER SO MUCH! I guess I won't be playing this game anymore. What a bummer.

The closest match to my issue that I could find suggested that this was due to an incompatible graphics adapter:

https://www.worldofgothic.com/gothic/?go=g1troubleshooting

"After the beginning of the game and the intro video, severe graphical errors occur, the landscape and objects flicker or partially disappear.
Severe graphical errors occur if an older ATI graphic adapter is installed. The problem is that these adapters are not completely compatible with DirectX.
Piranha Bytes works on a solution to this problem along with ATI."

The computer I was using has an Intel UHD graphics card, not an older ATI graphics adapter, but I tried installing Gothic 1 Union on a laptop with an nVidia card and it works great.

Call me backwards, but I actually preferred the way the non-Union version ran, because the Union version won't save my video preferences and the keyboard controls are a bit different. But at least I was able to finish the game.

I don't know if savegames are compatible between versions but...

If you use GOG Galaxy you can go back to the previous version (right click on game, "Manage installation" > "Configure" > choose version) - anyway I think it's better that you download and store also the previous version file as a backup for the future cause you should find the pre-UNION version as a downloadable file in EXTRAS section of GOG website and GOG Galaxy.

GOG has Offline Installers of the latest version of games, I store them, and install from these files, and with GOG Galaxy you can also disable the automatic updates to avoid unwanted changes.