Several regulations that apply:
135.207 - night, visual surface light reference sufficient to safely control the helicopter.
135.159 – helicopters with a gross weight of 6,000 lbs. or less that carry passengers under VFR at night need to have a slip skid indicator, artificial horizon and a directional gryo.
Under 135.159 you can carry cargo only and would not need any of that equipment.
Operating at night in the GOM would surely be a fertile environment for spatial disorientation.
Hopefully pilots that fly at night in the GOM or elsewhere have an instrument rating but it’s not required by the regulations. Even having an instrument rating is no guarantee if one does not keep his/her currency.