I replaced the gas cap but I am still getting the same message.
check engine light is on. I get a message to check gas cap. on 2004 Pontiac Grand Prix
If the lamp has not gone out after a few days you will need to have a service shop or dealer run a diagnostic test on the fuel evaporative system. The onboard fuel emmisions system test itself for possible evaporative leaks in the fuel tank and lines. It has turned the light on because a sensor indicates the system will not hold pressure. This is not a safety issue...and emmisions issue.