Yes the tailgate is metal, however there is a raised panel that goes along the bottom of the tailgate that is painted plastic. The plastic would break or crack rather than dent, so a dent would mean it's in the metal part of the tailgate and not the plastic trim.
Unless you do body work, or don't care about the looks, take it to one of those "Paintless Dent Repair" places. They can do wonders with most dents like that.
I had a 1997 Pontiac Grand Prix that encountered a hail storm. My insurance paid me almost $700 for repairs but my agent recommended that I go to one of these dent removal specialist. I did and it only cost me $400, and he removed all the hail dents, plus he even removed all the minor ripples and stretch marks in the body that were caused when the panels where stamped out at the factory. When he was done it looked like a show car....The body was flawless.
...Rich