Codes P0455 and P0456 EVAP

Neither of those codes indicate a bad gas cap. They do indicate that the EVAP system has detected both a large (P0455) and a small (P0456) leak. Could it be a bad gas cap seal? Maybe, but that is easy to check. Remove the gas cap. Is the area it seals against on the fill neck nice and clean with no rust? Look at the o-ring on the cap itself, is it soft and pliable and not cracked or hard? Apply a thin coat of petroleum jelly to the cap seal area and put the cap on. That will provide a seal. Then drive the car, if it is the cap, the light should go off when it runs the EVAP test, that can take a couple days though.
If the light does not go off after a week it is not the cap.