Hole in hull

Is it a hole all the way through the hull or jut the gel coat missing?

A hole completely through will require some reinforement on the inside with a layer or two of fiberglass cloth and I suggest epoxy resin. Then the hole itself is filled with thickened resin leaving room at the surface for gel coat. Sometimes it's easier it fill the hole completely and then route it out with the Dremel for the gel.

If this hole is just in the gel then get one of the gel repair kits available at marine supplies.