There is a GMC dealer near the Randolph, NJ inspection station so I went in there. Gas cap was $19 bucks. I would have been happy to pay it. They didn't have a '91 gas cap understandable. Ran around and got a Stant at Advance. $10 -ish.
I usually douse such rusty situations in Rustoleum Rusty Metal Primer. I think it used to have fish oil in it!
You can cut it 50% with paint thinner and then do a second coat. Helps for a while. I still like Rockauto.com. When I got a new gas tank for The Big Bopper, I painted it up good with Rusty Metal Primer. The rear end was really getting away from me too, the u-bolts and all that area. I just chipped and wire brushed away as best as possible and brushed it heavy. I better go out and see how it's holding up.
My '93 Ranger frequently kicks off the pump. In NJ you have to have a third world non-English speaking pump attendant pump your gas. I always say, "Sorry it always does that..."
Along these lines, EPA rules now demand that even 5 gallon gas cans have inoperable badly designed spouts! Go on Ebay and look at the prices of vintage gas cans!!!! The best design I ever saw is a Blitz brand that has a flexible spout that just pulls up out of the screw cap.
Ain't the smartest city on the planet ....
-- For the record--best to do something then nothing. Every little effort helps. Preventive has been workin for me for years & has saved me a lot of money & aggravation 
That old one looks like my first season with a steel chimney pipe.