ScotchBrite pads.
These are synthetic polishing or blending pads similar to the green pot scrubbing pads sold in grocery stores.
You can buy them in finer "grits" at automotive supply houses.
Use the pad to rub the scratch until it's removed or at least blended in, then finish off by "stroking" the pad in the same direction that the gun's original grained finish ran.
This will restore the factory finish and blend the area with the rest of the gun.
For a shinier look, you can use a metal polish like Mother's Mag polish to give the gun a shine.
Be aware that you can't use this method on the bead blasted areas like the top strap.
Only bead blasting can restore those areas.
