I think it varies by country. I know that when i am in a saw shop in Canada all of the laminated bars and most of the solid bars are oregons. in the states the are all german made Stihl bars. FWIW i would not waste my money on a Oregon or GB made stihl. You can buy both brands cheaper when not branded Stihl.
Never seen a GB bar made with stihl markings on it, but I can understand why they'd have GB making em for the austrialian market, GB makes bars in austrialia, its cheaper than having windsor bars shipped from way up in canada to another continent.