If you're keeping the gas water heater in the garage(?) a line-voltage thermostat that goes that low (such as the Honeywell T6051A, which goes as low as 46F) running a small potable water compatible circulation pump for some baseboard would get you there. Without knowing the actual heat load of the garage at 50F (you'd have to do some load calculations) it's hard to say for sure how much baseboard it would take. In some locations the heating loop would have to be isolated from the potable supply to meet code, in which case you'd be adding a plate type heat exchanger, a second pump, and a small expansion tank for the isolated loop.
If the garage door is insulated, with reasonable weatherstripping, and R13 fiberglass in the walls the load at 50F is going to be pretty small, and you might consider an electric heaters sized for the load running off a line voltage thermostat. The T6051A is good for 16A @ 120V, which ~1900 watts (=~6500 BTU/hr ). That's enough to heat a somewhat leaky 2x4/R13 room that size to 50F @ 0F outside, and probably enough to cover 50F at your coldest winter temperature. The standby losses of a gas water heater would be supplying some of the heat.
A ridiculously oversized gas-burner with a blower such as a
Reznor that can use a wall thermostat would work with the Honeywell CT50K1028/E thermostat which can be set as low as 35F. That solution would have the advantage of being able to heat up the garage quickly if you even needed/wanted to. (There are other gas-burners that could work too- that was just one random selection.)
