None other. It's actually quite offensive in that context..
Mong is short for mongoloid, or mongolism, which is literally an older/alternative term for Down's Syndrome, also known as Trisomy 21.
It's the meaning of the term, and the origin of the term. The fact people don't know this doesn't mean Ricky Gervais doesn't. He might seem stupid, but he almost certainly knows that. I don't find him amusing in the slightest, but that has nothing to do with this. It has to do with the fact he isn't funny.