Straightening a bent tail gate, ideas?

This is what I would do. Take your tailgate off, lay it somewhere flat, then put a 2x4 across it and try progressively hitting it harder with a sledgehammer or standing on it or something. With a blanket or something in between if you don't want to wreck your paint.
That is, if you just want it 'more or less' straight and lining up and you don't care about having it perfect. Otherwise just take it to a body shop.

 

I would be hesitant to try the pull it straight with a ratchet strap, but it *could* work. The problem I'd see is where to hook the strap. It's all sheet metal, which, by itself is not very strong. The strength comes in the shape it's in. When you hook a strap to it, you're putting a point load on where ever you're hooking it to, so I think you'd more likely bend whatever you're hooking to, rather than re-shaping the entire structure.

Maybe instead of hooking something to the tailgate itself, rig up a 2x6 to the back of the gate, then attach the ratchet strap to that, instead of directly to the gate. If you have a ball hitch on your bumper, use that to pivot the 2x6 against, so a majority of the load is being applied to the top of the gate where it's bent (I assume that's where it's bent).

I would also rig something up to where you are using both of the tie-downs at the front of the bed so you distribute the load evenly between the 2 points, but also so you are pulling straight in, rather than to one corner.