I hope you ordered one that has a RESIDENTIAL USE sticker on it. A two car garage usually takes a 45,000 BTU unit. Also, don't forget the flexible gas connector. It's code.
Make sure parts are available locally. If you purchase it from a home improvement store, they will not have any parts. I have heard of people waiting over 3 weeks for parts.
The Vantage 2 IS NOT approved for residential use. In order for a heater to be used in your personal garage it MUST be approved for residential use. Robert Gordan does make a Caribe residential garage heater and Superior Radiant also makes a garage heater called the GR45.
If there is a fire...