Several common abrasive systems can be used, some such as Farecla Automobile range (G3, G6 etc) are more or less the same as the Farecla marine range (Profile 100, 200 etc) and there are loads of other similar products available. The main thing with an abrasive is to use the correct one , and if the scratches require a coarse abrasive then they must be followed up with a finer compound.
Most important after doing all the cutting back work (and often forgotten) is the wax application. Several coats are best, the wax does not require to be anything special, simple GRP moulding hard wax will do, but it must be left on to dry before buffing bac and several coats applied.
This is bac breaking work and there is no easy way to do it. However if this is done the Gelcoat will be back like new and it will be protected from oxidization by the wax that you have put on.
Good luck !