If it was the gas cap, the light will go off on its own. usually takes 2-3 fill ups.
One thing you can do is go to the local Autozone and have them check the codes for free. Get the codes and post them. Autozone will also clear your CEL light for free if you ask them to.
Or just buy one while your there. They range from $20.00 to $3,000.00. Just get one that can clear the codes. An OBDII scanner will read codes for any car built '96(?) or later. If I were you, I'd just buy one. Chances are excellent you'll need it eventually.