Like with any installation, garage gas water heater installation must be performed by following certain set of rules. You should always check those requirements with your local building department but assuming that you are asking about a regular tank type gas water heater you can follow this link - http://checkthishouse.com/5712/garage-gas-hot-water-heater-raised-above-the-floor-fvir-safety.html to my article which explains a typical garage water heater installation.
The garage doors are not required but there must be sufficient amount of fresh air (combustion air) supply to the WH installation location - more details here - http://checkthishouse.com/2531/combustion-air-for-fuel-burning-appliances.html
Let me know if you have more questions but please provide details about your installation such as: garage square footage, type of water heater, BTU's, etc. (everything listed in my articles).
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