If you're talking about an old military gas can, I tried a silicone gasket years ago. It sealed well enough, but it was too soft, and it couldn't handle the twisting that was part of closing the cap. Gaskets are available on-line from military surplus dealers, but if you buy any, be sure they are still soft. Gaskets get harder as they age, and they won't seal worth a hoot.
Short term might be some cork or fiber gasket material. I've cut similar gaskets using a chunk of plywood chucked up, and then staple the material to the plywood, making sure that the staples are outside the gasket area. Grind a lathe bit into a knife shape, and cut the center out, then cut the outer diameter. Don't try to use an Xacto-knife blade or a utility knife blade - they don't survive for very long.
Military gas cans normally need a tool to tighten - the standard 18" long heavy-duty screwdriver works well.
FYI: In WWII, there was a shortage of tankers (due to German submarines), and the military had a hard time shipping gasoline to Europe. Since the military also needed gas cans, some genius got the idea to ship filled gas cans by liberty ship. The cans never seem to seal 100% - about 98% is about as good as it ever gets. The end result was that they had an entire ship full of 5-gallon cans, with every one of them leaking just a little bit as the ship rolled and tossed. One captain reported that the ship's bilge pumps were always pumping gas along with normal bilge water, and that the "smoking room" was a lifeboat that was towed behind the ship. Also remember that Liberty ships were steam powered. Sounds like an interesting way to make a living....