How do I fix gutter nails that are coming out?
If the nails are loose and pull out easily, just pounding them back in won't help much.  

The best way to fix any kind of nail/screw hole where the screw or nail won't grip any more is like this:


Remove the screw/nail.
Take some toothpicks and dip them in wood glue (e.g. tightbond).
Stuff as many toothpicks into the hole as you can, snapping them off flush with the hole.
Put the nail/screw back in (while the glue is still wet).


I find myself doing this most for loose door hinges, but it works for pretty much anything.  Once the glue dries, the result is better than new.