Simple and quick demonstration on How to Remove GMC Chevrolet Truck Tailgate. Here is an overall summary but each individual step by step picture below contains an explanation.
Step one: Lift handle on tailgate and open to about 50%-75% open. This will allow for some slack to be left in the attaching cables.
Step two: Depress the clip (outboard towards inboard) on the cable end where it attaches to the truck. Slide clip off trucks' metal dowel . This is most easily done with a flathead screwdriver.
Step three: Line up lower bottom insertion slot on tailgate to where opening on slot allows for removal and pull it off.
After doing this one time, you can have one off in about 3 seconds. While convenient for us it is also convenient for tailgate thieves. It's a good idea to look into tailgate locks available on Amazon. I'll post the link below but also you can use hose clamps as I demonstrate here:
Amazon link to Tailgate keeper (paid link): https://amzn.to/2TBqcCJ
This demo was done on a Silverado but it will work for other models of trucks such as GMC Sierra's or Chevy Colorado's just to name a couple.
To start this quick and easy tailgate removal process you need to lift up on the back handle. If the handle is broken (I've had broken ones before), you need to get in there with a pair of pliers and pull on the engaging rods within the tailgate to release it.
Fold the tailgate so it is about fifty percent lowered or as some may call it, in the half cocked position. It doesn't have to be perfect but the idea is to lower it enough so there is slack on the connection cables on either side.
At the end of each cable is a sliding lock piece with a clip. You must lift up on the clip and bring it over the top of the bolt. I like to use a flathead screwdriver to push this clip up. It can be done with a thumbnail but in my opinion easier with a flathead to pry it apart a little. Some of these can be very stubborn or rusty and stiff.
Then push the locking piece up so the head of the bolt can be pulled through the larger opening in the locking piece at the end of the tailgate cables.
Once the cables are loose on both sides, fold the tailgate down to the fully open position.
Once it's folded down to the fully open position the tailgate will pull straight off.
The "C" cup and the tailgate bolts on the truck are purposefully oblong by design to keep the tailgate from bouncing off.