Water heater tanks, and all new heaters, has been a big issue in the last few years. Mainly complicated by Covid and the mad rush to fix up old campers. Late last year, finding a new water heater did start to gain some relief. The prices were all over the map... big time.
A few things, yes, sadly Atwood Mobile sold out I think 4 to 5 years back now. Dometic picked up the water heaters, ranges, furnaces and the power jacks (tongue). The jacks were sold off to Lippert who then just about discontinued all of them.
Dometic is still selling some parts for the Atwood furnaces, water heaters and ranges, at least for now. They have discontinued many low volume selling parts.
The tank service kit for your G6A-3 (I think you mixed up the letters in your post) is listed as Atwood 91412. From my places I buy parts from, these 2 list it for sale, but for sure call them to make sure the inventory is still in stock. Many of my other go too places were out of stock.
Tweety's, I have bought from them in the past with a positive experience https://tweetys.com/atwood-dometic-9...ater-tank.aspx
RVParts country is somewhat new, I have not yet bought from them. https://www.rvpartscountry.com/91412-inner-tank.html
From what I know, the 91059 tank you are looking at, is a gas/electric combo tank that fits the electronic ignition and where the control board runs the electric element portion. That tank does not have the tapped hole for your older all gas valve sensor probe. This tank will not work from what I know in your case.
Since yours was made and as the years went by, the gas only Atwood heater was converted to gas with electronic ignition. There is no pilot light and no sensor probe that screwed into the outside area of the tank that works with your old gas valve/controller.
So now what?
I see you found the all new Suburban option that was made to fit into the camper opening of the Atwood heaters. https://suburbanrv.com/water-heating...-water-heater/
That is an good heater, just some do not like the steel tank with the anode requirement in the RV setting. The anode is in the drain plug hole and some of us drain the water heater after every trip. Which means taking that anode in and out all the time increases the odds you will cross thread the anode in the tank and then deal with that issue as time goes on. And there is no quick drain feature with a valve that we could do with the Atwood's.
If you do go the Suburban route, consider piping in a drain line into the cold water fill port on the back of the tank. This might allow the anode to stay in the heater and still be able to drain it. I have seen some older Suburban's in motor homes that did that, but the not sure the newer heaters have the cold port in the low spot on the back of the tank.
I have a friend up the road with a new Keystone camper with the Suburban in it and he has to deal with that anode/draining issue as they dump the water every campout.
I'm not saying not to get the Suburban, just understand the drain/anode issue if that is a concern for you. There are a lot of campers out there with the Suburban. How people deal with the fresh water system in their camper is a whole topic by itself.
There is another option for you. Dometic redesigned the Atwood style heater and as of late last year, you can find it for sale. These are the 3 models they now offer.
I bought 2 of 6 gallon gas/combo electric ones in Jan time frame of this year. And they where made here in the US, at least for now. It still uses the aluminum tank concept, consider it an updated design of the last Atwood's made. I installed one so far and it is up and operational in a friend of mine's camper. And it does fit in the hole in the camper the old Atwood came out of, but you have to do a little wood work to narrow the hole as the new one is square. They sell a door kit that fits over the new wood to make a conversion from the old Atwood to the new Dometic possible and deal with the siding.
I was going to do a post on it, but have not made it to it yet of the new Dometic option. If this option is of interest to you, let me know I'll get the post up and going.
Here are a few quick picks from the install.
New heater test fit in the hole in the camper
Wood work to close up the width
The door retro fit kit installed
The retro fit kit door closed
Regardless if you go with a new Suburban or Dometic, you will need to deal with water, gas piping changes and electric changes. You need 12 VDC to run the controls and if you want the electric option, you need to run 120 VAC to the heater.
The cost of a new replacement tank is not far away from an all new heater if you shop around. Just you have to deal the the piping, gas and controls changes.
Hope this helps