Detergent cleaners are sold in every chandler, I find they work quite well but of course are somewhat limited in removing years of caked grime and fallout. My own preference is for Black Streak Remover, a product based upon EDTA. This is a harmless but powerful compound that is also used as a food additive. It is extremely effective in removing muck of all sorts. You will also find that products containing oxalic acid, e.g. Teak Cleaner, will work very well.
I have just used some stuff called "Y10" containing, I believe, Oxalic acid (less than 5%), on my boat with very good results. Expensive, about £9 from most chandlers; I expect an equivalent could be made at home. "Y10" says on the side of the pot that it will not harm GRP