I noticed something while tooling around the internet -- Steam gets a mention on the Arx Fatalis Wikipedia page as a distributor of AF. Why shouldn't GOG get a mention, too? I'm no Wikipedia editor-type, so would anyone like to volunteer?

True, but your edits get reverted back if you didn't join Wiki.
To answer OPs question: No GOG shouldn't be denied, but Wiki wars will keep odd facts like that out of articles.