320 grit sand paper allows for a fine sanding before you compound the repaired area. The compound will get the shine back into the gel coat. Start with a medium compound and then finish with a micro finish compound. The best time to do these steps are after the gel has cured, but while it is still soft as it will take less work to get it done. This is someplace between one and three hours depending on your mix and the temperature.
Final Repair with cut out below. The way you can see the repair is that it is not scratched up like the rest of the hull. Give it some time and it will match well.