GTO is based on the Australian Holden monaro coupe, you can see this.

http://www.1-18thmodelcars.co.uk/Motor%20Show/Vauxhall.htm

with some cosmetic adaptations to "pontiac looking" the car is the new GTO.
so, the reason for what the cars of the same brand look similar its to lower production costs, continue the same design tendecy for familiar looking, and keep the pontiac enthusiast consumers.

_________________________________________________ Contour 98.5 GL V6 Manual, 115,000 KM (actual modifications) magnaflow exhaust K&N induction kit, sport chip, resonator removed. iridium plugs, heat deflector,nology wires, no pre-cats.