If it is stained (ex. from boating in dirty water, letting boat sit in water overnight?) like Jman says, I have used FSR in the past, it works great. Then put on a good coat of wax afterwards. Your local marine store should have it. I got mine at West Marine.
I also used a bleach bathroom cleaner on an old I/O that I had with white gelcoat, not sure if it was the right thing to do? But worked at the time.
If it is oxidized from sun, use Robert's approach. Only reason I chime in, I thought I had oxidation and the gelcoat was actually stained from dirty water.