Heavy cut compound by hand (unless you have a rotary buffer if you did you wouldn't be asking lol). Use a liberal amount and work horizontally in the scratch with a cheap (or old) microfiber towel.. If the scratch is to deep to completely remove, the white will be cut out of the scratch leaving it almost undetectable. The good news it's off the crown of the truck... where your eyes focus on the vehicle, unless you're a midget.
If you're the OCD type and can't compound to satisfaction, see someone like me...
a body/paint man - slash detailer :endofshow