Very over-the-top, wouldn't recommend except for maybe cool screenshots or something ;)
just drop "system" in main Gothic 2 folder.

An extra vivid blue version which has no alpha channel (this does not seem to affect the game/sky in any bad way from what i've seen).
See screenshot. Drag drop the system folder into your main Gothic 2 folder and click yes to overwrite.

2048x2048 (other files are 1024x1024)
This one is the closest to the original sky though a bit more busy with clouds.

1. Extract archive, check screenshot in archive and select your favorite

2. rename the favorite of the .dds file to starsh (.dds)

3. file goes into system\gd3d11\textures (overwrite)

Done - you have a new night sky!