Vinegar is an exceedingly weak "acid" and if it does not cause harm to humans, doubt gel-coat would be adversely impacted. Hard water stains are caused by mineral deposits which result when the water evaporates to dryness leaving the minerals (such as iron, calcium, magnesium) behind. The stronger the acid, the more quickly the mineral deposit will dissolve and the less required.
I have used muriatic acid (HCl), a strong acid on gelcoat with no adverse outcome. Lime-away is phosphoric acid which is "less strong" and also works well. Both are much "stronger acids" than vinegar. Simply apply, let minerals dissolve and rinse.
Heck, as a reference, Coke has a pH of 3.65 and vinegar has a pH of 2 to 3...both are "weak acids" despite the pH. You can ingest both with no consequence. Lime-away has a pH of 1.2 to 2.0, but is a much "stronger acid so hence will dissolve minerals more quickly. It is the "alkalinity reducing ability" of the acid which determines strength, not pH. Maybe TMI, sorry!