I have a number of gelcoat chips on my deck -- normally from air pockets in the fiberglass behind the gelcoat.  I have learned to do pretty good repairs in the white gelcoat as most of those surfaces are flat, and it is relatively easy to sand smooth after curing.
Repairing my grey anti-skid is much harder.  I bought Gibco mold with the proper diamond mold pattern, but have often had problems getting the proper amount of gelcoat under the mold, or slight pattern mis-matches.
Recently I made some oak wood sanding blocks that make repair finisihing easier.  I took some 3/4 inch thick oak and cut 22.5 degreee of each side, so I had a 135 degree point on the center of the edge -- the same angle as the grooves between the diamond hatch.  Now, after I repair, I wrap my sanding block with wet or dry sandpaper and start in the non-repaired grooves and then go over the repair area. You can round the point at the edge of the block to make a ramp as the block enters the repair area.
For very small repairs, I no longer use the mold.  I just put excess gelcoat in the repair area, and sand the proper diamond pattern back in.
I have attached a picture that may help.  I hope this helps you as much as it as me.