Evap code?

[99Sierra_Z71_5.3]

I have a 99 GMC sierra with the 5.3 and the SES light is on. I had the code checked and the guy said it was the same code you get if you leave the gas cap off or loose. I know the gas cap is on tight and my question is what could be my problem. There is a noticeable loss of power in the truck and the light is getting aggravating. The guy at AutoZone also said a vacuum line could cause the problem so my question is where should I start looking? Any help would be appreciated Thanks.

[GM-Tech]

If it's not the gas cap the next most likely area is the evap vent valve. This does NOT cause a loss of power or any noticeable driveability problem. If you have a loss of power, it ain't because of the evap problem.