When I put the key in ignition it starts and when I take my foot off the gas it stalls

There is no 1.8L for this vehicle. If you have the 3.4L engine then replacing the idle air control valve should fix the problem. If you have the 3.6L engine then have your mechanic clean the throttle body since the 3.6L is drive by wire system.

After cleaning the throttle he will need a compatible scanner to relearn the idle. Sometimes the worse case scenario if it will not learn the idle then an ECM software update will need to be done so verify with your mechanic that he can actually do the repair. If he cannot then you will need to find another shop that could.

Try cleaning throttle plate on both sides and IAC hole with choke cleaner. if it continues check fuel pressure with a gauge auto parts rent it. here is an article on it.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator