in the realm of add on lenses for the G series i have to say i tried one, and i didnt really like it. it was the official canon wide angle. it wasnt full frame or anything. i assume the telephoto ones dont have this problem. But your best bet is to take your camera down to a camera shop and see for your self...
As for the lenses themselves, im not very educated on the ones for G series cameras, i assume the canon ones are nice though.