On several occasions over the years I've paid GM service to work on the tailgate handle....they didn't really fix anything...it never got better...only worse...and in fact, after pulling the thing apart tonight myself it doesn't even look like they lubed the one and only moving point that would have kept it in good working order (centre, eh)

You have to unscrewed the tailgate cap with a torx bit, same one for the liner. Remove the single bolt and then you can then pry the housing off the other side. There's three clips - get a flashlight and look through the holes for them. A flat-head wrapped with electrical tape will help you pry without damaging the paint. It's fairly easy to pull off with fingers once the pry gives you a tiny bit of leverage. Once that's gone remove the other two bolts to free the assembly from the tail. Then note the colored clips holding the rods to it. They will flick down easily (lots of YouTube of tailgate handle will show them flicking them down with a screw driver) and then you just pull the rod towards you out of their holes. You can then remove the assembly from the tailgate.

Go clean it - it's probably a mess. Then lube the centre rotating point from all sides, and maybe the left and right points where the handle itself actually moves. Spread and contract the left/right arms connected to that pivot point repeatedly to work in the oil. I literally did this rapidly more times than the silly things been used for real. Add more oil...and then refill your beer. Clean the tailgate while you are waiting for the oil to penetrate. Once you feel the movement getting easier re-insert it in the tailgate, clip the rods back in and bolt it back on hand tight. Try closing and opening. Should feel way better than before.

The single bolt has to be removed to reinstall the outing housing. Get it all finger tight and test again. Then tighten it up, replace the liner (cleaning the bolt heads as you go) and the cap.

Magic and a couple of pints. Why I had to PAY GM service to not do that properly the THREE TIMES I mentioned it wasn't working right over the eight years I've had this... When you get the assembly out if you can see rust looking down vertically on the centre pivot point that's because they did not lube it when you were PAYING for the regular scheduled maintenance. Thanks GM.