Gilben and Jeff share the same birthday, as they had both had their parties on the same day, which happened in "The Big Petey Pizza Problem".
Although most of the time Gilben doesn't move, in "Clarence's Millions", he is seen eating stickers and saying "Whoa!".
In "Detention", Gilben moves his mouth when the kids open up the cabinet full of toys.
In "Clarence Loves Shoopy", he moved again, while Ms. Shoop had chased everyone out of her house, Gilben is seen turning and running like everyone else did.
Gilben may be able to know that he is being watched. And as said before, it seems he is psychicly communicating with the other characters, but not you, as you can't hear what he is saying and all other sounds go quiet when he 'speaks'.