Hmmm.... where does one find the game folder itself? I looked in GOG /DOSBOX/ GABRIEL KNIGHT ETC program folders and I see no config file!? Thx
If you have the game installed you'll find the config file in the game folder, look for the 'dosboxGK.conf' file.
Open the file with notepad and you'll find the scaler line under the [render] section. The commented section of the render section shows what values you can play around with.
Here's the render section from my dosboxGK.conf
# frameskip -- How many frames DOSBox skips before drawing one.
# aspect -- Do aspect correction, if your output method doesn't support scaling this can slow things down!.
# scaler -- Scaler used to enlarge/enhance low resolution modes.
# Supported are none,normal2x,normal3x,advmame2x,advmame3x,hq2x,hq3x,
# If forced is appended (like scaler=hq2x forced), the scaler will be used
# even if the result might not be desired.