Join Date: Mar 2010
Location: Southwest PA
Drives: 10 Optima LX, 95 Astro AWD, 03 Monte Carlo
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You have a restriction in your tank vent, which won't let the air escape as you fill, and causes the gas to come back out the fill neck. Could be something as simple as spider webs in the vent line, or worst case, bad rollover valve which would need the fuel tank replaced.
If the gas cap was bad, it would cause a DTC for a leak. You probably had a P0449, which is a vent valve circuit or restriction. Autozone saw "Evap" in the code description and instantly sold you a gas cap. I'd take it back to them and get a refund.