[Trayder]

It depends if cured or not-

Vinylester resins utilize a polyester resin type of cross-linking molecules and if applied "wet to wet" on poly gelcoat it will chemically bond. (like in a mold)

If you are doing a repair with vinylester resin, you should use laminating resin. Laminating resin stays tackyas it will not fully cure when exposed to air, this will allow you to get a chemical bond between the final layer of gel coat with wax or gel coat covered by PVA.

If the vinylester resin has cured, rough it up and shoot your gel-coat, the mechanical bond will be more than suitable.