You didn't mention one very important detail: what type of a gas water heater you have and what type are you planning to get?
Power vented tank (with motor blower on top and PVC pipe) - more details are here / can be vented horizontally - http://checkthishouse.com/2991/water-heater-pvc-vent-pipe-and-power-venting.html
Most common natural draft tank (with draft hood on top) - requires a chimney - http://checkthishouse.com/49/water-heater-vent-pipe.html
Tankless - based on manufacturer's requirements, in most cases horizontal venting is OK
Direct vent type (tank and tankless) - horizontal and / or vertical depending on model / manufacturer.
Let me know if you have more question but please, be specific / provide detailed information about this appliance. Just click "comment" below.