A bit before my time and its not a feature I notice too much although it does make the side panels cleaner in appearance. The models GM did this on that I recall were there larger cars most notably the Chev Caprice from the 1980s era. I don't think any GM models still have the gas cap there now. Here is a Pontiac with the gas behind the license plate.
Early 1980's Oldsmobile Cutlass have the filler behind the license plate.
When it changed, I don't know.