Keep a segment of polyethylene tubing with a teflon check valve ( the former available at Home Depot et. al. and the latter available at many environmental service supply stores). With the check valve in the end (it should screw in tightly), you can pump any liquid with an up and down motion. With gasoline, once it's started, if you keep the can lower than the tank it will just gravity feed (raise it above the level of the tank to stop it).
