How far off are we talking? 1/4" , 1/2" ..or 1" at the top? Lay the tailgate on the concrete and verify it's good or if the new tailgate is 'tweaked' (warped).
If your new tailgate is 'true' or you want to use it anyway, no reason you couldn't do a little opening of the bottom hinge holes ....on the appropriate side, using a die grinder. With them slotted it would allow you to move one hinge 'out' so the top will go in. Of course the bottom of the tailgate might protrude a little bit, but down low it will not be visible to most onlookers (than the top). If your tailgate hinge on the body-side has a peg, just cut the peg and re-weld it back to the new location it needs to be (for strength) ...even if the peg is off-center.
If your tailgate is 'true' it's possible your your truck box was pushed in at the top or bottom from an accident, or a box side replaced with new at one time due to a collision ..and is a little off.
But if you have access to some tools or a buddy's tools and a welder, should be able to compensate if it's not grossly off. Good luck!