Just my 2 cents here -- obviously, the official answer is Foxeye's
-- but IMO, in a case where you based your work on some other piece of art (whether it's fan-art or not), but now you can't find the original to get the artist's name... I would think it'd be okay to post it, with a note basically to that effect. "I based this on a fan-art piece I saw long ago, but I can't remember the artist's name and can't find the piece now -- does it look familiar to anyone?"
That way, at least your viewers would know what they are looking at (they would know that some part of what they're seeing is from you, and some part is owed to this prior artist). My viewpoint is that giving credit to other artists is very important, but it's equally important that you're informing your viewers of what they are seeing and appreciating. (You don't want your viewers praising you for your handling of a pose, or a wonderful background, if that's what you actually got from the prior artist.) And you never know, maybe someone else will recognize it and be able to tell you who did it.
I'll be interested to see what Foxeye says, though. I could understand the gallery's rules going the other way.