I'm all for the tire-gate. But, what's wrong with designing something to attach to the bedsides?
I think the gate needs to be connected to the body and not the bumper. Adding separate steel uprights to the bumper to make a side-ways hinging gate seems a tad "clunky" to me. It also takes away some usable entry space into the bed. But most of all... it makes it impossible to quickly switch back to a factory tail-gate. ...because the steel structure is blocking the stock tailgate attachment points. I don't know about you guys...but, I could see myself switching back & forth with the stock tailgate from time to time. Those steel uprights would definitely need to punch through the top of the bumper and be gusseted from below, and even then...they might tend to deflect with a 100lb. tire on them. They would also need to be fairly thick-walled. When its all done...it will be 200+ lbs, and look like an afterthought. So for all these reasons...my vote is for something smaller, lighter, that works with the stock tailgate attachment points. It would be easier to make, and it would take full advantage of the attachment points that GM put there in the first place...and best of all, you can swap to a stock tailgate without a bunch of drama.
The stock tailgate comes off in about 30 seconds. Once its off, you should be able to fab-up a tube-steel frame very quickly that hinges, shuts, and latches just like the stock one. Then, all you need to do is "fill-in" the middle with whatever tire mount you want. Round 1.75" OD DOM tubing is the way to go, and you should be able to get away with .120" wall-thickness. Mitered square tubing may be easier for the backyard guy...but there is no way to make it look truly "professional". Even when you do a great job...square tubing still looks home-made. You need a bender to make it look sweet.
Another tip?....go to a junkyard and grab a spare tire bracket off a wrecked H3 SUV rear hatch door. Its stamped steel and bolts-on, so all you would need to do is weld a plate to your framework at the angle you want...drill four holes...then bolt the SUV tire mount to the gate. Done. You might want to pop a weld to the backside of the three studs first...as they are known to spin in the holes of the bracket, making it a real beyotch to remove the spare.
