There is a good chance that the linkage from the actuator to the latch has become unhooked somehow. This is not an uncommon problem. Once it is fixed it would be wise to safety wire it so it can't happen again. The hard part is that you are going to have to remove the trim panel to get inside the gate and see what is going on. About the best I can do to help is to send you the instructions from the GM service manual for removing the trim. I really hope this will help. They are just little push in pins, don't lose any if you can avoid it as they are not a real common size and may be difficult to find replacements for. Instructions below. I wish you luck with it. Thanks, Bob
Remove the endgate trim panel (3) from the endgate panel by gently prying loose the plastic retainers which secure the panel.
Install the endgate trim panel (3) to the endgate panel. Ensure that the plastic retainers are fully engaged.
Hm, Now that is strange, for some reason they do not show a picture of it. Only of the lock cylinder and some of the linkage. I will post what it shows but don't think it can do much except maybe give you and idea of what is on the inside. But even without a picture, once you get inside it will pretty much explain itself. Just look at the linkage and maybe if needed have someone work the handle while you watch and you will figure it right out just from looking. I also see a few other things that are not shown. You may find that part of it is covered by a piece of tin when you get inside also. Sorry, it has been some time since I have had one of these apart, but I am confident once you do see it you won't have any problems.