Exhaust stains can turn a white transom gray and make an otherwise clean boat look dirty.
As you probably already know, exhaust stains don't always come off in the wash. Some spray cleaners are strong enough to remove exhaust stains; however, if they're strong enough for that job, then they're probably also strong enough to eventually strip that area of wax, only making it harder to clean the exhaust stains off over time.
The best way to remove exhaust stains from gelcoat is to wax them off. This is something that can easily be done by hand with cleaner wax. Use a terrycloth rag to apply the wax, and wipe or rub it in until the exhaust stains are gone. Then use a microfiber rag to wipe the hazy wax residue off. If the exhaust stains cover a large area, you'll want to use several terrycloth rags as you go, so you're not rubbing the exhaust soot from the rag back onto another section of your boat.
When you're finished, you'll be left with a clean, white surface, and it should be a little easier to wipe exhaust stains off next time because they'll be sitting on top of freshly waxed gelcoat.