This appears to be the status that I'm at with Arx Fatalis thus far, as well.
I, too, purchased the GOG version of Arx Fatalis. I installed the program yesterday without any difficulty.
The game does play, but as Clocknova posted, any time there is text on the screen (like during character creation) the game drops down to about 5% of its normal speed. The performance is so poor that the mouse cursor jumps all over the place while attempting to carefully maneuver it around the screen.
I checked the logs and there are a large number of errors related to the DIB engine. An example being:
fixme:bitblt:X11DRV_ClientSideDIBCopy potential optimization: client-side compressed DIB copy
The log is spammed with hundreds, if not thousands, of these messages.
The game does, however, work. Just very, very slowly when having to deal with the way the game handles fonts (and possibly other screen overlays).
The only other problem that I've encountered is that the game crashes whenever I attempt to set resolution above 1024x768. In theory (and in practice according to posters on the GOG forum and elsewhere on the internet) the game fully supports a variety of resolutions, including widescreen ones. In fact, the game offers several higher and widescreen resolutions in its graphical configuration dropdown list of options. Selecting higher modes, such as 1152x720 or 1280x800 lead to the following error (and immediate crash):
1. The screen starts flickering quickly between the normal screen and a vertically inverted one of the same image.
2. This error message is then posted to the log, around 30 times:
fixme:d3d_texture:basetexture_apply_state_changes Unrecognized or unsupported MAGFILTER* value 4
Other than those two issues, the game does run so long as you keep it in 800x600 or 1024x768 resolution, and can suffer through the painful slowdowns associated with having text on screen.
Following is a copy of the full log, in case there's anything of value in it for troubleshooting:
Thank you very much to anyone who might have ideas on how to improve how the program functions, or address any of the err: or fixme: issues in the log.

This critically acclaimed first-person RPG takes the player on an amazing journey into the fantasy world of Arx. The game mixes intelligent story with immersive and actual medieval surroundings. Arx Fatalis allows the player to feel that all his actions have a direct consequence on those around him.
The goal of the game is to destroy the secret cult of Akbaa, the god of destruction. As a player, you must prevent the return of such a cruel creature to the physical world. Should you fail, Akbaa is bound to impose its terror upon the world of Arx.
You will become the hero of an underground world - with weapons, spells and magical items to defend against goblins, trolls, rat-men and other vicious opponents.
Besides brute force, stealth and the 'onscreen gesture based magic system' are the three alternative ways to succeed. Using the mouse, flaming runes can be drawn in mid-air. A combination of these gestures can create powerful magic spells that will protect you or empower you to defeat opponents and pursue the quest.