Better for the Marines to wear their Blues in and out but shuck down to their skivies and put on the gown to receive their diplomas. Disguising ones uniform by putting a tablecloth on over it can get you suspected as a spy and getting one summarily shot. Don’t disguise the uni.
This issue has been going on for years. Marine Corps doesn’t have an “official” policy beyond regs for the wearing of the various versions of the approved uniforms.
Cap and gown is NOT an approved uniform accessory.