Check Engine Soon - 2000 BMC Sierra 6cy

2000 GMC Sierra with 102K mi. Check Engine Soon light first came on when I filled the gas tank very full. When the tank got low, it started missing and the light started to blink. I assumed bad gas. Filled up again and added fuel injector cleaner. The light went out. Next tank, the light came back on when the tank got low. After fill up, the light went out. Now the light is back on even after fill-up. Now, when I remove the gas cap, there seems to be significant pressure inside the tank because you can hear air escape when you loosen the cap. Could this be a simple gas cap problem? Any other suggestions before I take to my mechanic? thanks

Welcome to AF. Get it scanned for the code first, sounds like the fuel pump may be getting weak or the filter is restricted, have the FP tested when the tank is low.