Well, I checked my console while carrying the guy around, and I noticed the game was rejecting all the milestones for other achievements (like, gravgun kills) and saying "Acheivements disabled, commentary on" or something. I'm left to assume I can't do the gnome one then.
Actually, the guy behind Concerned has already been mucking about with this, swapping out the gnome for Gordon in Ep 2. I've only found the directory with the files so far; don't know if there's a blog post or something. I can't see it, but I'm pretty dense.