Mineral stains are a lot tougher to remove than mineral deposits (water spots) baked on the surface so most off the shelf waterspot removers won't touch it. Because gelcoat is porous you need something that will effectively cleanse the area such as an acid wash or wetsand/buff through the effected layer of gelcoat if you really want it to look like new. Then once clean, follow up with a quality silicone wax sealant to prevent it from happening again.
Once sealed, the red clay will wipe off as opposed to staining the gelcoat.
FYI - carnuba wax leaves a beautiful, shiny finish immediately after application but even the high quality products tend to wash off after only a few uses or days in a slip leaving the bare gelcoat unprotected. Silicone sealants hold up in the marine environment which is why that's all we'll trust in our 'Sauce' line of boat care products