2004 Pontiac grand prix P0442 code

A while back my check engine light came on and I started getting "gas cap removed" warnings on my dash display. This lasted a couple weeks with me retightening the gas cap and the warning would go away. But the check engine light would remain on. Now for the last month the check engine light has been on but I have seen no gas cap warnings. I just failed a smog check with a code of P0442. If the reason my engine light came on was the gas cap, should it go off by itself or do the codes need to be cleared? Also if I clear the codes, do i have to drive for a certain amount of miles without the light coming back on for me to pass smog?

have you tried replacing the gas cap?

Just changed gas cap. Check engine light still on and today dash display said "check gas cap" with the old one, replaced and display still says "check gas cap"

So you need to get it smoke testted to find the system leak!