Fix for sagging gate hitting concrete
You could use an Anti Sag Gate Kit. 

Installing this will help to pull the corner up but i would rebuild the bottom rail of this gate first. 

I would remove the gate from the fence and place it on a flat work surface, loosen up  or remove the screws holding the main frame rails together and use a square to square it up, then use a gate hardware kit to put it all back together (  i would probably still use the cable kit for added support and adjustablity. )