I have a closet in my garage which currently contains my water shutoff valve and a natural gas water heater. I'm planning on adding a whole-house water filter that requires a little bit of power (36w max) so I put a GFCI outlet in the closet and ran 12/2 NM cable up into the attic. Now, there's a closet on the other side of the wall where my outlet is that contains two power circuits, but they're used for a different purpose (media closet) so I was planning on tying into a outlet in the attic that's off of the branch circuit marked "garage" on my breaker box. Does this make the most logical sense? It's a bit more work but it seems the most logically consistent to me. My garage is supposedly GFCI protected as well, but I can't be 100% sure since I don't have a wiring diagram, hence why I added another GFCI outlet in the circuit. Am I off in my thinking on this? Edit: I also put the outlet maybe 3-4 feet off the ground just to add in a safety margin for the natural gas, in case there's a leak.
For a load so small as < 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.