Alright, so this discussion has gotten me curious regarding fuel resistance… I've done a bit of digging on this on the paint side from info provided by the two paint makers, Rustoleum and SprayMax, as well as the BASF Handbook on Basics of Coating Technology. Pardon me while I reveal my true side for a moment… (by day I'm a nerd) I hope maybe I can help someone with this question regarding solvent resistance that isn't content without asking "Why?" as I personally dislike coming across posts where someone says, "well I've heard such and such to be true…" yet has no supporting data or evidence. That's how rumors and misinformation spread
Fuel resistance comes largely from a paint's ability to be crosslinked via reaction from individual polymer chains which are easily dissolvable into a network of molecules all tied together that cannot be easily dissolved by solvents (ie. fuel.)
Rustoleum's Engine Enamel is an enamel paint that uses a "modified alkyd" as its base polymer. Enamel paints will usually be crosslinked (ie. cured) via oxidative crosslinking. This reaction needs oxygen (or another oxidizer) to be carried out. This is largely why you want to spray lots of thin coats over a span of time. If you spray too thick, the top of the paint will cure, or skin over, and prevent the lower layers from curing and they will remain soft and less chemically resistant. Rustoleum's Engine Enamel tech data sheet lists their resin as a "modified alkyd" which is very unspecific, but the fact that it is regarded as modified tells me that it was better solvent resistance than a standard enamel as there are some chain modifications that be made to tune the resin to have higher heat and solvent resistance. Their product is essentially a 1 part, where the crosslinking component comes from the air (oxygen.)
The SprayMax 2k clear coat uses acrylic resins as the base, which are mixed with isocyanates when you put the button on the bottom of the can to "activate" it. When you do that, you're allowing the acrylic resins and isocyanates (the crosslinker in this case) to react with each other and start crosslinking to form a larger network of urethane polymers. (BTW, isocyanates are very nasty and they are happy to react with your body as much as your paint's resins. You can become sensitized to them with prolonged exposure and get a nasty, itchy skin rash so it's a really good idea to use respirators and keep the mist off your body.) The 2k is a 2 part because the base resin and crosslinker in the same can, but separated until you puncture the divider and mix them together.
Ultimately, urethanes are going to be more resilient to solvent exposure than their enamel counterparts. If you are able to get the 2k I'd say go for that for your gas tank. If not, you should still be just fine with the Engine Enamel, but make sure you have a rag handy and wipe up any drips as quickly and gently as possible. The enamel is designed for intermittent, quick exposure whereas the urethane can withstand exposure longer.
With regards to baking painted parts, there are a couple things to consider… If the manufacturer specifically calls for heat curing then it's likely because there is a set of reactions that cannot occur without the elevated temperature. In that case, do it absolutely! The paint won’t be as strong without baking.
Otherwise, cooking a painted part that isn't designed with high temp. reactions will allow crosslinked bonds to form faster. Reaction rates, in addition to drying rates, are very sensitive to temperature (see the Arrhenius equation) so higher heat will cure paint faster.
BUT, it doesn't necessarily mean it will be stronger than an unbaked part, and if you bake it too soon, it can lead to the lower layers being weaker. This is because you can skin over the top of the paint before the lower layers have had a chance to react with oxygen. Skinning over means you've crosslinked the top layer tightly, but when you do this, it becomes less permeable to oxygen which is needed to cure the lower layers. In other words, if you paint it thick and put right in the oven it could actually take longer to cure. Let me quote the BASF Handbook on Basics of Coating Technology which in reference to enamel paints says, "During oxidative crosslinking of the unmodified alkyd resins the forming film undergoes molecular enlargement caused by crosslinking, starting from its surface. A reduction in oxygen permeability is an associated feature. Good through-hardening therefore requires only slow surface hardening."
Again, this is why people say, "use many light coats."
Sorry for the length, but I hope this is useful for someone without a reactions background that has the question of gas resistance on 2k vs. enamel in the future and values content over opinions and hearsay.