Bit open ended question IMO.
A start would be ; heat loss calculations
how many years occupancy for this purpose.
Is the gas (meter etc) inthe garage ? Then a gas convector would be good. Just needs a small flue on an outside wall.
Portable gas heater.What is the best option for heating a garage being turned into an office?
The garage is separate from the house so connecting to the boiler would be tricky.
Portable gas heater.