|You may be able to buy something that is advertised as the appropriate colour, but remember that your interior colour will have shifted with time, so the match may not be quite what you want.
Another solution is to have the present colour analyzed and matched. Many (not all) larger suppliers of fibreglassing supplies have a scanner that can read a patch of old gelcoat and spit out a recipe for the standard colourants required to make that shade (paint stores can do the same, but you'll get paint colourants, not gelcoat). it's not cheap, because you have to buy a fairly large quantity of gelcoat to get the mix right. In addition, some proprietors are capable of looking at a sample and mixing a very good approximation; this may be a better route as they may not ask you to buy 10 lifetimes' supply of the mix, unlike above.
C&C 27 Mk IV