the patterning and colouring, the shape of the head as I think the Gibby's have a wider head and seem to have more bristles on the side of the head.
and gibby's have the great sail fin they raise like in the picture commons dont have such a big back fin. commons seem slightly more tolerent as well as I find my gibby used to get stroppy when there was fish bigger or near his size housed with him but he is great with any other fish as long as he is the biggest.