Despite the website has gone defunct, DarkXL still works for me. So awesome!
I can confirm that DarkXL works great with the GOG version of Dark Forces! I was playing it yesterday, and having a blast with it. Here's what I did:
1. Download and install XL Engine.
2. Open the XL Engine launcher. Click the Dark Forces icon on the bar on the left.
3. Point it to your Dark Forces directory.*
4. Click Play. This launches DarkXL.
*The first few times I ran DarkXL, all I got was an unresponsive white screen. I then copied my Dark Forces installation somewhere else on my hard drive, pointed DarkXL to that, and it worked. My guess is the path was too long, so if you can't get it to work, maybe try that.
For what it's worth, there does not seem to be anything unusual about the GOG version of the game. As far as I can tell, it's basically the contents of the DARK directory on the original CD, along with a pre-configured version of DOSBOX for launching the game. So, in theory, anything that requires a copy of the game should work with the GOG version.