Sounds like maybe a EVAP Vent Valve. GM has issues with them. Usually they get plugged up with dirt and mud and cannot close all the way. This causes the small leak.
EVAP leaks are determined by monitoring the pressure in the gas tank by the pressure sensor that is located on the gas tank. If the pressure loss is above a certain amount, then you will get the small leak code, if it is a larger loss, then you will get a large leak.