The gas cap is not the only possible cause , this code shows a large evap system leak.
Other potential causes are as follows.
If in fact the gas cap was the problem the code will reset itself after the next test cycle
runs . Here are the parameters for the test to run.
You can reset with a scan tool , the large parts chains will usually scan and clear
the codes for free. Also check the o'ring on the gas cap for cracks.