That looks like a nut was tightened on without a washer. All that gouging should be on the washer, not the dropout. I like to use the axle-nuts with the built-in captured washers. Apply a little oil between the nut and washer and this will ensure that the washer stays put on the dropout without spinning.

You can repair the gouge marks by using a precision file and carefully filing in towards the slot to prevent any damage to clean paint. Then touch up with nail-polish.