96 GMC Jimmy

Car has ~150,000 mile on it and normally runs great. However, in the past year I have started having problems with it not starting after it has rained, or when I havent driven it for a week or so. I can not figure out what the problem is. The distributer cap has been replaced twice and the dealership replaced the intake manifold gasket. Neither of those fixes did anything for the problem. I just got a new battery for it, but the problem remains…any suggestions?

Does it turn over? If so you need to find out if it is spark or fuel trouble.
Starting fluid or a spark checker or you can use timing light.
After you find out at least you know witch way to go.

Find out what type injection, you have the problematic Central Port with its poppet valves and the much more reliable multi-port.