Scrub with bon-ami, especially good for the textured surface on deck tops. If you think the gelcoat has wax on it, (and at some point someone probably waxed them) use a product specifically made for wax removal. Then you can wet sand (by hand) with 400, then 1000 grit sandpaper and then use a fine grit (automotive type) rubbing compound and a buffing wheel. Finish off with 303 UV protectorant.
Captain Dave has a whole bunch of past posts (H16) that are more in-depth. Jamie ZX as well.
Skip any steps you want, except wax removal, you'll grind the old remnants into the gelcoat.
hobiejohn at earthlink dot net