Other than stripping it and repainting it with a catalyzed alkyd base paint, there is no way to "make" conventionally available paints fuel resistant. Some catalyzed clearcoats could potentially help, but they make things look like brilliant plastic, and you would have to make sure there were no "witness lines" to allow fuel to attack the finish under the edge of clearcoat where the clearcoat stops.
Catalyzed/activated paints are not something to be handled lightly, and not something to be sprayed in an enclosed garage without adequate ventillation, and good personal protection/remote air source.
You might want to consider one of those gas cans that do not let gas flow until a lever is pulled, that way you minimize the risk to your new paint.
A few years ago I painted a gasoline tank with rattle-can red. Afterwards, I took extreme measures to make sure I didn't slop any gasoline onto the tank.
In spite of the care taken, as time went by the tank has suffered a number of spills. Much to my surprise, the paint wasn't affected one bit.
Now, the sad part of the story. I don't remember what kind of paint I used; however, I believe it was Rustoleum. I've only used three brands of paint: 1) Rustoleum, 2) Krylon, 3) K-Mart generic. I suspect it was either 1 or 2.
One of those rattle-can varieties of paint will resist fuel spills. You might want to experiment.
It has been my experience that paint spills don't usually bother thoroughly cured paint. It is the soaking of gasoline that lifts most all paints but the two part urethanes. A leaky fitting, a bad fill cap gasket, or a stuck float in carb causes long term exposure of the paint to gasoline. Not many paints will hold up to this. I have had good luck on one engine with clear coat of spray shellac to a frequently soaked area.
the main thing is try not to get a hard edge around the top of the fuel neck. That is normally the problem because the gas will get under the paint if so. Any catalyzed paint will hold up well after allowing the proper dry times and if careful on that paint edge on filler neck.