Unless you're actually measuring something with any importance, a gallon is about 4 liters. More closely 3.78541... But saying a quart and a liter are about equal is often good enough.
You can centrifuge it. Heavier molecules move closer to the bottom. Repeat until you reach the desired purity.
You need shitloads of ordinary water to extract a liter of heavy water. And I'd be surprised if there's any superheavy water in that, because tritium radioactively decays.
Fun fact: Heavy water is slightly toxic to humans. If you only drank heavy water, you'd die as it shifts the chemical reactions in your body out of balance. A moderate amount is fine though.
Emphasis on the slightly. I believe it would take weeks of only consuming heavy water. You could certainly drink a single glass of it without ill effects.
IIR, a number of rich & famous people have developed severe mercury poisoning after extended conspicuous consumption of swordfish and other very expensive/very contaminated seafood.
Obviously the luxury bottled water industry is missing out on the potential of D2O and T2O.
Probably deuterium. Half-life doesn't only mean radioactive decay, it also means the rate at which a medication is broken down or excreted from the body. Tritium is more likely to be used as a radioactive tracer.