Adding an outlet in a water heater closet
For a load so small as &lt; 50 watts, I wouldn't worry about what circuit I was adding it to.  I'd recommend using the closest one easily accessible.

Before spending the money to buy a GFCI outlet, I'd check to see if the location I'm tapping into was already GFCI protected.  There are GFCI testers out there, and I'd recommend using one if you have one.  If not, use a small incandescent lamp that you can switch on and off without touching any metalwork of the lamp; and a discarded grounded (3-prong) plug.  Turn the lamp off and temporarily connect the two prongs of the lamp to the ground and hot wires of the 3-prong plug.

Plug it in to the source outlet and briefly turn it on and off.  That will safely trip its GFCI protection if it has GFCI protection.  Probably quickly enough that you won't even see the lamp light up.  If the lamp lights up, then your source isn't gfci protected and it's worth the money to use a GFCI outlet.  I'd recommend adding that outlet in a position as near the circuit breaker panel as possible to get the best utilization of it though.  So replace the most upstream outlet and use the one you pulled out of the wall, in the new location.

Put the outlet at a height that is convenient and accessible.  The risk with a gas leak is if there is a spark and that spark occurs in an area of atmosphere with the proper mixture of gas and oxygen for ignition.  A plug in an outlet is't going to make sparks just sitting there.  It's only a risk when plugging and unplugging.