How can I treat a gasoline spill on my black cotton and spandex yoga pants?
This is such an interesting question, and the answer needs to address more than just cleaning—there’s a safety issue, too. Clothes washer and dryer manufacturers don’t want oil or gasoline soaked items going into their appliances for cleaning because of the risk of spontaneous combustion, and they put a warning on the appliances and in the user’s guide to that effect. Throwing away oil-saturated clothes is the safest course of action—check with your garbage service provider to see what they recommend for safe disposal before tossing any items away.
This isn’t very helpful, I realize, and that frustrates me—I also know that presoaking or pretreating with a stronger degreaser (like Goo Gone or DeSolveIt, available at hardware stores) could be effective at removing residual gasoline. You would need to test a hidden part of the pants for colorfastness first, and then follow the manufacturer’s directions. If you know how the gasoline got onto the pants, and how much of it there was originally, that would also be helpful in deciding how to proceed.
Even so, not knowing enough about your specific situation makes recommending discarding the pants necessary from a safety standpoint.