I'm pretty sure that this is a 1982. That was the 1st year for Motobecane's inexternally brazed frames and the only year that they used a set screw for the seat post on traditional round seat tubes, though there was an associated casting (see attached). In 1983 they went to a more traditional binder bolt, located beneath the junction of the seat stay and seat tube, though they did retain the set screw on frames with non-round, aero seat tubes.

To date, all the Motobecane serial numbers that I've seen have been seven, numeric characters, so the the G-prefix is puzzling.