I'm interested in buying this version, but I have one outstanding question: Does it come with any fixes to get the multiplayer to work?

I don't recall the exact terms, but I DO recall that Warcraft II uses some particular network protocol that Windows phased out after Windows XP, thus the multiplayer won't run on any later operating systems.
I remember reading about this, and there were ways to get around this using other programs, but I never understood how to get those running properly. And for some reason my old CD version wasn't running properly on my old XP computers I keep stored in my closet.

But now we have this GOG version.

Does this GOG version of Warcraft 2 have those various details set up already so that I can play a LAN game on my more modern computers? Can I run a LAN game with this version out-of-the-box? Or at the very least, are there some easy-to-follow instructions on how to get it running?

Yes. It includes a wrapper that makes the game think it's using the old (unsupported) network protocol but it actually uses UDP. There's no setup necessary.

That's what I was hoping to hear! Thank you!

Perhaps more important, how's the multiplayer population? If there's no one to play with it's not worth much after all.

Does Warcraft 2 allow a Direct IP game?