There is a very split second where Gilbert rankles with the idea of being made fun of for his accent. Then he looks and-

...

Changelings are infertile, there is absolutely no way that that man reproduced in all his cavorting. Right?

So it has to be someone else who may have been at the table. Simple enough- if George could arrive with him, so could someone else, even if they hadn't met.

"If you're certain!" he chimes, because he might not have been a Guest but like hell he's bloody serving one, "quite a gathering, isn't it?"