Stockpiling gasoline is not illegal as long as you've bought the gasoline legally.
However cities and towns can have restrictions on how much gasoline you can store on your property.
Some cities and towns may have limits and restrictions that say you can only stockpile or store 25 gallons on your residential property.
This mainly applies to gasoline stored in gas cans or in a steel drum or external fuel storage tank and not your vehicles gasoline tank.
You could legally keep more gasoline in your fuel storage tanks.
However while you could legally store gasoline or stockpile gasoline at your home or business, it's not a good thing to do as gasoline degrades over time and usually within a month of it not being used, especially without fuel stabilizers.
If you do stockpile some gasoline, you should use it as you keep it and then replenish it with fresh as gasoline goes stale.
Diesel fuel lasts longer, but gasoline does not store very well for very long and has to be used up before it goes bad.
As gasoline sits, the gasoline starts to turn to a varnish and become unusable and if you use too old of gasoline in your vehicle it can clog fuel lines, fuel filters and even fuel injectors and cause issues.
If you use old gasoline in a lawn mower or other small engine it can result in the carburetor clogging up and lead to repairs needed.