The tailgate is thicker gage steel than body panels, and is very difficult to pull or hammer out dents. If you're talking rust, and limited to a small area, then repair is best option, if larger area, I'd suggest replacement. In the past, I bought a patch panel to replace the entire rolled bottom portion, and it was a lot of work to cut weld, and body work. In retrospect, I should have bought new and saved the time. Also, I've learned that it is easiest to drill holes from the back of the tailgate and insert a drift pin to hammer out dents to the front side - a common area for dents is the spare tire carrier latch where it gets slammed and overtime dents the tailgate pretty good. Once panel is straight, plug weld the hole closed.
Hope this helps.