Most abrasive cleansers will dull the gloss of gelcoat somewhat. Bon Ami is a unique product in that it contains the abrasive feldspar rather than hydrated silica, which is in most other cleansers and toothpaste, so it should be safer than Comet or Ajax, though still not ideal. For the best cleaning and improved gloss, use a good rubbing compound like 3M Microfinishing Compound. Use the liquid version with a rotary buffer and the paste version by hand. An orbital buffer is not suitable for this task.
Jimbo