I have had a couple of plastic gas cans over the years whose fill spout nuts had cracked.
Sometimes you can find replacements, sometimes not.
Other times you have to buy the entire nozzle assembly, which is often expensive and may not be the same style as the original.
And yet other times the can has been scrapped just because of the cracked nut.

I have a favorite five gallon gas can whose clamp nut cracked as you can see in the photos.
It is a Blitz brand, who very sadly are no longer in business, and I have two others by them that will also receive printed nuts when (not if) theirs fail.
The original nut was somewhat poorly designed to be honest and was far too thin and weak at at the top where the clamp flange met the threaded portion.

I saw other designs on this website but none of them were quite what I wanted.
So I designed this one.
The hardest part in the process was the choice of material.
ABS would have been more pliable, but is not known to hold up all that well with gasoline exposure.
Therefore I chose PLA.
Upon closer examination however I discovered that the seal ring on the nozzle body itself prevents gasoline vapors from getting to the nut anyway, so in the end either material would probably have worked just fine.
Nylon or PETG would work too if you happen to have it available.

You will need to be careful about material shrinkage because it is quite a tight fit to begin with.
You may want to print a "test" nut with a very low percentage of fill and thin walls.
See how it fits and then adjust things in your slicer accordingly to compensate for shrinkage and thread pitch in the final print.

This particular nut is very strong and has held up for over five and a half years now (as of February 2023) with no signs of change or deterioration.
And because I designed it with large gripper ribs it is actually easier to tighten and remove than the original was.
Another one I made for a second Blitz can whose nut also cracked has also held up very well.

I hope this helps you.

P.S.- it was fascinating to watch the threads being printed.

Slight Update- 18MAY2024
I have added a slightly modified version ("NewGasCanNut").
I made the changes because it better fits onto yet another Bliss gas can I have.
Its OEM nut finally cracked recently and this new version fits perfectly.
The differences are as follows-
Original TopID 1.500" Thickness 1.063" ThreadRootDia 1.632"
New TopID 1.400" Thickness 1.013" ThreadRootDia 1.682"
Coincidentally, I tried it on the original gas can (the one in the photos) and it fits quite nicely as well.