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just need to mention this:

1- Open regedit and navigate to HKEY_CURRENT_USER\Software\Sierra On-Line\Gabriel Knight 3\Engine .

2- Edit the decimal value of these keys :
(default) = yyyy
Screen Height = yyyy
Screen Widht = xxxx

Where xxxx is desired width & yyyy is desired height.

(default) and "Screen Height" MUST have the same value.

not sure how well known this solution is...
fischkopf: I have now found a 1080p solution for older graphic DirectX 10 cards as well:

Get DxWnd, unzip to in a separate folder. Apply the regfix from my archive. Now open DxWnd as admin (this is important, otherwise it won't save the profile, next time you open it) go to File / Import and import the GK3 profile from the archive. Now you need to change the file paths, because the profile uses my paths. Right click on the profile and go to "modify". Change the paths so they match yours in the "launch" line, add this after the .exe (without quotes):

" -width 1920 -height 1080"

Now you can play GK3 in 1080p with your DirectX 10 graphics card.

This has a few downsides compared to the DirectX 11 method:

You need to run DxWnd everytime you want to run the game (no big deal I think)
ReShade doesn't work (you can force AA and Anisotropic filtering in your drivers anyway)
The mouse will leave trails in 2D menus (map screen, main menu, inventory, Sidney)

I wasn't able to fix the mouse trails, but I think this is still much better than playing the game in a much lower resolution. :)
The link to zippyshare is broken.