A gas piston system adds only a few ounces to the rifle, 2 or 3 at the most.
I suspect that the piston alone would weigh more than 3 ounces, but that’s okay. After all, Eugene Stoner did adopt the piston design in his later AR designs (which were in turn copied in rifles such as the Daewoo). In truth, the G36 is just a modernized AR-18/180.