I downloaded King's Quest 8 from here on GOG.com, through the Galaxy client, last night.
The application started up just fine, but there were issues.
The first one I experienced was a taskbar overlap and the title bar of the window the program is contained in also overlapping.
When I tried to replay the intro, which didn't ever play, so who knows in what context replay means, the program crashed back to the Galaxy client.
I selected new game, and it started copying files to my hard drive. I find this odd, since files are already on the hard drive. This is what happened with the original program. I don't understand it, but there it is. About 1/6 of the way through the process, the program crashes back to the Galaxy client.
I tried changing my display settings on my computer to 800x600, the lowest they can go. I launched the game, and there was no taskbar overlap, nor was there a title bar overlap. I felt I was making progress. I clicked on the replay intro button, and I began to hear audio. Good. Where was the video? I didn't see any.
There was something about IV5 and a decompressor missing. I still can't start a new game.

I am running Windows 10
The Nico site was down when I tried earlier, so no chance of those files being downloaded or used.

What I am asking for is a build/install of King's Quest 8 without these issues, so it can be played properly.

This solution works and it was posted after I made mine:

VanKhron: Problems solved! Firstly, the "Eternity CD not found" problem seemed to be caused by the dgVooDoo wrapper. Playing with it's settings and dlls a bit fixed the issue (it seems like you should use the Napalm 'Glide3x.dll' file instead of the normal one).

As for my main initial issue. Some searching around old KQ8 topics led me to believe that the install directory might be the issue. So when I re-installed the game, I used the old default folder from the original game (C:\SIERRA\Mask\GAME) instead of GoG's default folder (C:\Program Files (x86)\GalaxyClient\Games\Kings Quest 8). This fixed the problem immediately.

So I must repeat. The GoG Galaxy Client's default install folder does not work. The directory name is either too long or uses illegal characters that the game can't understand ( I'm looking at you, '(x86)').

Hopefully this post will help people enjoy this game in the future.

(It seems like I apparently can't mark my own posts as solutions. That's dumb.)
AAHHH thank you ! I had the same problem but followed your instructions. Much much much love !! finally i get to play Kq8 without virtualbox.

(end copy and paste)
I copied and pasted the folder in the C:/ directory as said. One other change I made was I changed the resolution of the computer display to the lowest available which is 800x600. The graphics option changed in the game for some reason when I did this, but I changed it back, and it worked. Make sure 3dfx and 800x600 are both tagged for this.
I was able to watch the intro movie. The game installed itself. I was able to play the game. I have only begun, but if there are any other issues, you'll surely hear them from me.