Like others said, glue all the way.

Check the link in my sig. I have a tailgate like yours with the whole thing missing. I have a bare bones truck, so it was probably swapped. But, it was nasty. Different color under the plastic, rusty, 9 holes in it, and bent. I'd think about unscrewing the plastic to see how your metal is before you glue the red back on. But yes, keep it if at all possible.