You can find the Gasoline in the generator shack to the west of Cabin B. Throw the Lantern, a Match or a Flare for a big explosion. You can also create a line of gasoline to act as a fuse. You can shoot the Gasoline with the shotgun or use a match. The explosion will also destroy locked and barricaded doors.
If you drop the crate on the Gasoline, it will not apply double damage to the killer.