Palomino Maverick M-800 on 2018 GMC 3500 STD BOX 4x4

I found no reference stating if an M-800 should go on a short bed or not. All the pictures I found seem show it on LB trucks. Even the TCM article doesn't elaborate: https://www.truckcampermagazine.com/news/tcm-exclusive-2010-palomino-maverick-m-800/.

Looking at specs, pictures and the floorplan, I believe it should work fine with your truck from a weight perspective. To know for sure, take your empty truck to a scale and get the actual axle weights. The difference between the RAWR on the door sticker and the empty weight of the rear axle will give you a good idea of the available payload in this scenario. Expect all of the camper weight to go to the back. If your truck has single rear wheels, be aware that the RAWR of your truck may be the sum of the load ratings of your rear tires, so if you overload the axle, you may overload the tires -which wouldn't be a good idea.
How well the camper holds up with more than 2ft sticking out in the back unsupported is another question. I have no idea.

I hope some more comments will come in.

Please excuse me if you know some or all of this already:

The CoG of the camper depends a lot on where the heavy items are - water and propane tanks, battery, appliances. Usually, it is somewhere around the first third from the front bulkhead. Obviously, the position of the actual CoG changes somewhat with the way you load the camper - where and how much you put inside.

In theory, if you were to jack the camper up at its actual CoG, you could balance it there.
If the camper CoG is exactly over the rear axle, ALL the weight of the camper goes to the rear axle and the weight on the front axle remains unchanged.
If the camper CoG is in front of the rear axle, some of the camper weight is transferred to the front. This is the preferred loading situation.
However, if the camper CoG is behind the rear axle, not only all the camper weight goes to the rear - some weight gets transferred away from the front and the axle gets lighter - which is not good for the handling of the rig on the road (in the extreme, the rear would tip over and the front would be lifted off the ground.)

First, you’re mistaken regarding CoG. It “could be” at the center longitudinally, but likely is father forward than that. CoG is where it is.
Regarding your other question, is it a short bed camper or not? At 9’ long, it would be one of, if not the longest short bed suitable campers out there.
Other than that, you have zero issue with weight.
And even if it’s not a SB camper, it’s very light weight for a camper that long. If it doesn’t have a rear overhang that would prohibit running with the tailgate on, I’d consider running it like that.
And even assuming if the CoG is a foot or so behind the axle, if you do the math it likely doesn’t remove enough weight off the front axle to be of worry.