When AC aligned their fare classes a few years back, most of them seemed to line up at that point, but with the subsequent changes and branded fares, they don't seem to line up as well as they did a few years ago.

I had a UA ticket last week with an AC segment, and the UA G fare had mapped into an AC K fare, so I'd expect your G fare may be the same. It showed as "Flex" when I'd looked it up with the AC PNR, but it credited as K for mileage and was considered as a K fare for eupgrade requests.