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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...