That depends on what codec variation the gateway uses. The best available GSM codec would probably require G.722 or G.711 to minimise degradation. Although, on its own, G.722 has a marginally better mean opinion score than G.711, I suspect that relates to its covering frequencies that GSM doesn’t cover, so the combination could be worse than G.711, and G.711 uses the same network bandwidth and less computing power to transcode it.
If your GSM network crams them in with the lowest bit rate option, it will make no differences as GSM will be the limiting factor.
Maybe you are failing to understand that GSM is always worse than G.711, not least because the internconnects will be in G.711, and therefore limited by that.
Remember that the MOS of a combination is likely to be worse than either in isolation, and won’t be better.