You first have to do your homework.
1. Compare the cost of your electric and gas utilities and determine the net cost per 1000 btuh. Generally, gas will win this comparison but not always, so do the comparison. If it's a tossup, equipment or installation requirements/costs may be the determiner
2. Perform a heat loss calculation to determine the required heating capacity.
3. You have two primary types of costs - capital (which includes the cost of the equipment including installation), and operating (which includes energy use, maintenance and depreciation). Insurance (confirm with your agent regarding combustion equipment), capital costs of money, etc are all about a wash for you, and normally would not be considered. If you are using this space on an occasional, as opposed to regular basis, your priorities will shift.
Perform a comparison of the various types of heat, based on the selected energy source, electric or gas. For example, if gas is the most efficient, you have the following choices:
Gas forced air. Typically a ceiling hung unit heater (or two), though a residential style furnace would allow ducting. Furnaces generally top out at about 100,000 btuh which may or may not be adequate for you.
Gas hot water. This may be in the form of a radiant floor, hot water baseboard, cabinet heaters, etc.
Gas radiant heat. These would be good choice, for example, if you generally work in only one area of the garage.
Each of the above have advantages and disadvantages - faster pick-up time with forced air, greater uniformity of heat with hot water, etc.
This will be a large expenditure so it will be worth your while to study the options, particularly in view of local requirements and standards. Once you have narrowed your options, especially energy source, a more detailed opinion can be offered.
As always, offered only as opinion