I have built two shops....both larger than the one you outlined. I insulated both. both had an insulated garage door on the end. In one, I put a heat pump that you might find in a hotel room.....single unit that does A/C and heat. Worked great. On the other, I installed a small heat pump similar to what you might find in a house. It was a lot of work (did it myself), but it worked well too. Now, I have a home in an airpark with a pretty good sized hangar....it is insulated.....I put the largest window unit / heat pump I could find and it does great.
So....for simplicity, I would recommend a wall unit similar to what you find in a hotel room....heat pump for both heating and cooling. If you really had your heart on using gas to heat, then I would install a separate, independent gas heater.
Ellis
Grumman Tiger Owner, A&P/IA, RV7a builder, Fayetteville NC