(fiberglass) the gel coating may have been worn off from age and using improper cleaning agents over the years. Once this gel coat wears off the only way to fix it is to have it professionally repaired and in many cases requires a new shower/tub base.

The fiberglass gel prevents the porous material under the surface from soaking up the moisture and soaps used in the shower. The result is the gray look you cannot get clean.

If the offending area is tile, or glazed steel/cast iron then using scrubbing bubbles, bleach or the other ideas would be the best. But understand some chemicals can etch the surface making it harder to keep clean. As the micro abrasions become deeper the soap scum fills them and it then takes longer and more elbow grease to remove this offending material that causes the base to darken.