Small evap leaks trigger the gas cap warning on cars that use the differentiator lamp (check gas cap) because it is a common failure mode. This reduces people having to come in for repair only to find out their gas cap is loose or leaking. So the CEL is not lit for small evap leaks when there is a CGC lamp.
Just like it takes several drive cycles to extinguish the diagnostic lamp, it will not illuminate unless the condition exists for several drive cycles to avoid false positive indications. Resetting the lamp by clearing codes or disconnecting the battery will not help and may only cause more grief when all of the other monitors also get reset.
Remove the gas cap and carefully examine the filler neck. Feel it with your finger nail. It should be baby skin smooth. Same for the mating gasket on the cap. Dirt on either mating surface, rust, pitting, cracking, hard gasket on the cap due to aging all will allow vapor to escape and set off the diagnostic indicator. If it seems OK, you can try generously applying some vaseline to the mating surface. If the lamp goes out after 5 drive cycles, it is likely the cap or neck. If not, there are other sources for small evap leaks…