So this sounds like a new stm32g4 subdirectory would be beneficial.
You would have to know how pervasive the differences between the chips are. The STM32F0 cohabitates quite nicely with G0 and L0, for example.
If, for example, all of the peripherals are the same, but there is a difference in clocking or power management (which would be my suspicion), then those could be handled with separate .c file selected in the Make.defs file.
You don't want to clutter the code, but neither do you want to duplicate all of the drivers. The first is ugly and effects usability; the second could be a LOT of work. You can't make that decision without understanding the differences and making some trade-off (even if it is still mostly subjective).
