Check Engine for Vapor Canister Valve and gas guage problems on 2002 Pontiac Grand Prix

Last year when I was parked on an incline, with about 1/4 tank of gas, my guage went to empty. As I drove away, my check engine light went on. It went off a couple days later but returned a couple days after that. Since then, my gas guage has not worked. Even when I have the tank full, any time I am stopped and idling, the guage goes to empty. More importantly, I had the check engine diagnosed. They stated it was a vapor canister valve. I am not sure of the code, but I thought it was a Po440. Do I just need to change the vapor canister, or with the gas guage doing this odd behavior, could it be something else to?

the gas guage and the 440 are separate issue.
the 440 is an evap code. there is a small leak or a solenoid that is not working. if you had it diaged, then get it repaired as it will put out the light for good.
as far as the gas gauge, the sender is bad and should be replaced. you cannot service the unit without removing the tank.
yes, replace the vapor canister valve, and you will most likely replace the fuel pump module.