When a single fish can be an inch or twenty inches, fish per gallon is not consistent enough to be a very useful stat depending on what you want to use it for.
"How many inches of adult fish per gallon?" would be a little better question, and gives a much different picture for most every tank presented. (If the idea is to use this info to avoid over-stocking your tank, use this measure and not simple inch/gal.)
I've got three adult Barnacle Blennies in a 40 gallon, FWIW. 3" total of adult fish in 40 gallons.
Just for reference, my old freshwater Oscar tank was two fish in a 75 gallon, or about 26" of adult fish.
-Matt
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