Back it up against a tree and hit the gas.

Back up an hit a tree did everybody laugh, Hope that made you feel good because somebody is struggling for help !

First thing I would do would be to go to a good hardware store and buy a "shoulder Bushing" that fits over the hinge post on the bottom and in the tailgate, to take up some of the slop and endplay. Then I'd straighten the bow out of the tailgate itself, and that will close much of the gap. You'll probably need to split the shoulder bushing to make it work/ go in place on the bottom.
If it were mine I'd get the shoulder bushing in place, stand a 4x4 up on the bumper in the middle of the tailgate, then back it into a tree slowly to take the bow out of the tailgate.

Or, you could go to church next Sunday, start banging on it in the parking lot after services, and you'll probably have half a dozen country boys/ farmers helping you fix it in no time!

As others have stated, my money is on the hinges being messed up. You can get them cheap on rock auto or a junkyard

You said you needed help with a WARPED tailgate.Then you said you just bought the tailgate three days ago.First off all,why would you buy a warped tailgate.Second of all,,if the tailgate is warped,,the only way to fix that gap is straighten out the tailgate.I was serious when I said back it against a tree and hit the gas.