Yeah, I have the same problem. Sadly it seems they have added the inferior version of the game with a lower sound quality, so we're not getting the best results.
Compare
How it is: http://www.youtube.com/watch?v=WULkhdQmJQw
How it should be: http://www.youtube.com/watch?v=nEg8DDg-BP0

The second youtube video is taken from the Windows version, which has the intro as an .avi file with its own sound. The in-game sound is identical, there is no version with superior sound quality.
They just didn't have that brilliant sound quality back in the day, with neither the storage/compression or the technology to get crystal clear sound.
In fact, if you open up the c:\program files\gog.com\gabriel knight\gk1.gog file with WinRAR/zip, you'll see that the .avi files are still present there, along with all the Windows executables.

Ok. But I played the game a couple of years ago, and I can't remember the sound being THAT bad.
Well, I'll play it without voices. Doesn't matter that much.

Oh, I see. Well, is there any way to incorporate those files instead? Seems like it shouldn't be difficult.
But on topic, I also played it about 2 years ago, and I definitely don't remember the sound quality being this poor. Maybe it requires some DOSbox tweaking?

The .avi files are only playable in the Windows version. There's really no easy way of getting them into the DOS version that GOG have setup.
However the .gog file is basically a disc image of the original CD, so it should be more than possible to extract that and run the Windows version if you so wish.
It will use the same resource.aud file, so the sound quality in-game won't change, just the cutscenes.
That said, I did always prefer the music in my Windows version, though that was probably just due to my soundcard.

OK, I started up the game again, and I'm certain that it's not supposed to sound like that. During the entire opening credits a loud distortion, lik an electric sparkle, is constantly on top. This distortion is also present with the voices, and with some other random sounds.
I don't know if the problem is with my computer or with the actual game file, but it's very annoying to the ears.

Can you access the original setup.exe (or install.exe, I'm not sure) in the GOG version? Maybe you can change the sound settings there to see if it helps.

Definitely sounds like problems with your setup. I don't have any sort of distortion like that.

No, that doesn't seem to be possible. I didn't get any choices when i installed it the first time, except for the file location of the game.
I do see a grapich mode setup icon in the game folder, but nothing for sound.
Maybe the game has bad compatibility with my soundcard?

You can probably find the install.exe file in the directory of the game. But first, maybe you should try to increase/decrease CPU cycles in DosBox.

Sound is completely fine for me too. Music and dialoges are clear, no distortion at all.
Most likely its your soundcard then.

Yeah I guess so.
What's strange is that the sound/music in the Sierra logo doesn't have any distortion at all. It starts when the game meny appears.
Also, the first minute or so of the intro movie doesn't distort. When the credits start, it does.

The credit music is digital music - everything up to that point is midi.

your right, I watched the intro again( turned sound up realy high) and the credits and menu music are distorted. But after game starts it dissapears for me and dialoges and game music are completely clear. I probably wouldnt even notice the menu part if you havent pointed it out.