Since you already have gas available, one of
these [
supplyhouse.com] would work well. It's probably got enough capacity to get the garage nice and toasty even in the coldest weather. Not the cheapest option to purchase, but would likely be cheaper to operate than an electric heater in the long run.
You'd need to run an exhaust vent out thru a wall, and gas piping to it. If you were to use any type of portable propane or natural gas heater, you need to be careful about the combustion products building up inside the garage. Also a floor mounted heater is in the danger zone to ignite any gasoline or other explosive vapors that settle near the floor.
If you give me the insulation thicknesses of the walls, ceiling and the garage door, along with door dimensions, I could run a heat loss estimate for you to size a heater to get to your 60F inside. Would need your location (closest big city) for outside design temperature also. It's what I used to do for a living.
Split system heat pumps don't put out much heat when it's frigid outside, but if you also need AC, that's an option. But you'd probably need supplemental heat in the coldest weather.
Another consideration with an electric heater is that you need to have the service size to support it. If it's going to suck enough amps to overload your service, that's going to get expensive.
