Whenever Ive repaired gelcoat, whether on boats or motorcycle fairings, ive found that using a hard rubber sanding block of the type used by car body repair shops (I have bought them from Halfords in the past) helps on larger areas to keep the surface flat. Use wet and dry paper in ever finer grits, from about a 280/320 working down to 800 or even 1200 with plenty of water along with the occasional rub of soap to prevent the paper clogging will, with lots of patience and care leave a surface that will polish easily to a good gloss finish. For best results dont move on to the next finer grit until all previous sanding marks have gone and if you notice a low spot developing, dont be tempted to keep working at it, but add another thin layer of gel coat. T-cut will do the final polishing job though products such as 3M Finesse-it designed for polishing 2-pack paints give a very high gloss, though are far easier to use with a machine polisher.
In practice, Malcolm, there isnt an easy or quick way to get the sort of finish you want. It's a laborious and time consuming process when done by hand
Steve