My son has a 2000 GXE that has developoed a stalling issue when decelerating and seems as if it wants to stall at idle as well. As soon as a little gas is applied, the idle appears normal.

He was very overdue for plugs, etc., so I changed those and noticed the gasket under the distributor cap is shot and also appears that there may be oil present under the distributor cap as well.

How difficult is it to change the distributor assembly? Is it possible to clean and repair the existing one? I did not want to tear into this until I had a chance to post about it here and also to acquire parts. The gasket does not appear to be available by itself. and the assembly is a bit pricey as well, so I would like to minimize the wallet pain as much as possible.

Thanks for any help in advance.

You may have 2 issues here.
If the oil is coming from the inside of the distributor (and it probably is) the seal inside for the shaft is worn and letting oil inside. It is a matter of time before the engine stops running or wont start. Replacing the distributor is easy, just make sure the rotor points to exactly the same place as when you took it out.
A lot of these cars have broken intake gaskets, usually near #4 port. Spray some carb cleaner around the area and if the idle smooths out, its broken.