Gel coat is more porous than acrylic. An example of this is how older fiberglass boats sitting for extended periods of time in water at a marina get blisters. Over time the wear and tear, especially on the bottom area, of gel coat showers and tub/wall units can get deep staining. A trick to getting out the stains is to soak the surface in Easy Off oven cleaner, then rinse. A word of caution here. Be certain the unit is indeed gel coat. If it is acrylic, the oven cleaner can cause irreparable chemical burns to the plastic. If the oven cleaner does not work, cut polish can be used although it is not very effective on textures bottoms. Cut polish, also called rubbing compound, can be purchased at stores that sell auto body supplies. Sometimes soaking the affected area in bleach overnight can also help.