Gasoline is produced in the Refining Forge. If you place 3 Oil and 5 Hides in the Refining Forge and light it up you'll get 5 Gasoline, cans included! As mentioned, Gasoline lasts a long time. I've found that the Generator and the Fabricator are quite fuel efficient.
For more information on finding and harvesting Oil and Hides, please see:
Hides & Harvesting
Oil & Harvesting