When I hear weird voltages and intermittent, I think LOOSE NEUTRAL.
The neutral bus in the panel that feeds the circuit your garage door is on is not securely attached to the utility company's neutral.
If you're lucky, the mistake is in a sub panel and easily fixed.
If not, it could be he main panel, though that is less likely because ground and neutrals are supposed to bond there.
It could also be behind the meter, or at the utility's transformer.
Go check some 'good outlets' that come from the same panel. If you see voltages higher than 120, or really any odd voltages, shut off the main breaker, and fix the neutral. A loose neutral causes voltages on each Hot to vary wildly depending on load, and can easily damage devices in your home.
With the meter pulled, I'd recommend you check all three lugs in the main panel, and in every sub-panel if you have them. I've heard stories of "licensed" electrician forgetting to tighten down the lugs. And once a service is live, nobody in their right mind is going to poke or prod at the service conductors in a panel. So it'd be hard to notice.
While the mater is out, a utility person will be there anyway. So ask him to check the utility's lugs too and check for a loose neutral on their side.
I'll also go on to note a couple mistakes in your original question that I'd like you to correct, or clarify.
The other outlets after the garage door with the 88v have less than a volt on the neutral side but good 120v between ground as well.
On the neutral side. What were you measuring? From Neutral to what?
120v between ground. Again, between ground and what?
WP outlet
What's that. Did you mean outdoor outlet with weather resistant cover?
it is testing good 120v between ground and neutral.
That wouldn't be good. That would be extremely bad. There should be negligible voltage between ground and neutral. Ideally none. Certainly less than 1-2 and then only in a very large house.
The other outlets after the garage door with the 88v have less than a volt on the neutral side but good 120v between ground as well.
Again. Incomplete picture here. When you give us a voltage, you need to say between what two points you were measuring.
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.
Best Answer
L6-30 is Hot Hot EarthGround. So no Neutral involved.
You're in luck. You shouldn't need to run a seperate neutral line.
You have two options if you want to add gfci protection to this outlet.
1) Replace the breaker that feeds it from the main panel with one that has GFCI protection. This is probably the best choice, because the protection would extend the entire distance of the buried? cable. It's the least amount of work and least expense. But it means if that GFCI trips you have to walk back to the house and go to the basement? to reset it.
2) Replace the outlet with a "GFCI Spa disconnect", and move the outlet slightly so that it's powered by the spa disconnect. It's rare to find a spa disconnect that comes with a 30 Amp breaker though. Most are larger.
Because you must not discard your pre-existing 30A breaker in the main panel, it would be safe to use a 60A GFCI Spa Disconnect. The GFCI capability would work regardless. Code permits the larger breaker downstream. But it might be confusing for those who come along later, so leave a note in/on the box if your spa disconnect is a higher amperage.
So If you take this option, I recommend finding a GFCI Spa Disconnect panel that does not come with a breaker, and additionally purchasing a 30A breaker for it. (Make sure its listed as compatible.)
3) Possible third option, do both options above. This protects the wiring between structures, as well as keeps you from walking all the way back to your basement if the fault was only local to the equipment in the Garage.
4) You may also be able to find or construct an "inline GFCI" that plugs into the L6-30 outlet and provides an L6-30 outlet. But they're going to be rare and not protect the buried cable.
GFCI breakers do not require access to a neutral to work. Only if the load requires a neutral does a GFCI also need access to neutral. If you don't intend to use a GCI breaker's neutral output lug, connect its neutral input pigtail to its neutral output lug so its obvious to anyone later that neutral is not used.
To provide Ground Fault Circuit Interruption (GFCI), all conductors intended to carry current must pass through and be disconnectable by the GFCI device.
The only reason many GFCI devices have a neutral is because 120v loads send current across neutral.
Most residential circuits are 120v only. Most residential 240v-capable outlets are actually 120/240 and have a neutral to provide the 120v service. But your L6-30 (Or any 6- for that matter) is not capable of providing for 120v devices, and this means your load doesn't use neutral. So its GFCI protector doesn't need access to neutral either.