OK, I faced a similar problem about two years ago when I went from an F-250 4X4, back to a gas engine F-150 4X4. I realize it's not a GM product, but the problem is the same. I have a Bombardier Quest 650 ATV and they are wider, longer, and heavier than your normal ATV of the same cc range. The back end of my Bomb hangs over onto my F-150 tailgate approx 10 inches and my Bomb weighs 850lbs, I believe.
I go to Colorado every year for first and second rifle season and my Bomb goes with me. I've had no problems with weright on the tailgate for the 14 hour trip, each way, BUT when you leave your tailgate down you must be mindful of road debris, stones, etc, hitting it and marking it up.
I checked and there was nothing on the market to "cover" and protect the tailgate. Cabelas has a product where you remove your tailgate and hook this "bed extension" into your tow hitch receiver and it supports the back end of your ATV. Weight concerns on your tailgate are of no concern at all then.
What I did, as I wasn't as concerned about the tailgate weight as much as I was about road debris marking up the down turned tailgate, was I bought one of Cabelas cargo extensions that also hook into the tow receiver and cut a piece of plywood to fit inside the cargo extension "tray". It sits about 8-10 inches below the lowered tailgate and has blocked/stopped all mud, stones, etc, from hitting the actual downed tailgate.
You might look into Cabelas ATV extensions. That would solve any concerns you have about your tailgate, I would think.