You don’t need to dilute a gallon of sea water at all, it is perfectly safe to dump it back into sea as is.
For you, locally, sure.
It'll kill however, plenty of things.
It won't kill anything. The surface of the ocean has natural fluctuations of salinity all the time due to evaporation, and it doesn't cause mass die offs. It's literally a necessary part of the ecosystem (and in fact discharging freshwater can cause problems to the ecosystem). But if your discharged water is within the natural 30 to 40 grams of salt per liter range, there's no ecological problem here.
It's not waving a word around. There's natural variations of salinity of oceanwater, roughly 30 to 40 grams of salt per liter. So diluting the discharged brine so it's within that natural evaporation-driven band would be perfectly safe as it's in the natural variation of salinity.
You can't just wave a word at it like dilution and think you're solving an engineering problem.
If you got 1 gallon out of sea water, what do you think you need to dilute it to be safe? Typically, it's 99%∆.