"MARGN" is the usual abbreviation of the time for "Marine Gunner", a warrant officer unique to the Marines. "MG" was perhaps intended as a clarification for readers who might not know the meaning of MARGN.

Warrant officers are generally intermediate between non-commissioned officers (NCOs, such as corporals, sergeants, etc.) and commissioned officers (lieutenants, captains, majors, colonels, etc.). They first appeared in the British Royal Navy, as technical specialists. Early warrant officers included masters (who managed the sailing of the ship), surgeons, pursers (who managed pay and supplies), carpenters (maintaining the ship) and gunners (maintaining the guns, and managing the operation of the guns).

Warrant officers were authorised for the US Marine Corps in 1916; the first USMC warrant officers were Marine Gunners and Quartermaster Clerks (approximately equivalent to RN gunners and pursers). The first USMC Marine Gunner appears to have been Henry Hulbert, a marine with 19 years in the USMC who had reached the highest NCO rank of sergeant major.

During World War II, in 1943, the USMC warrant officer titles were changed to "warrant officer" and "commissioned warrant officer" by Congress, to match the Army. Since then, the title "Marine Gunner" has returned. Currently, "Marine Gunner" is a title; their official rank is Chief Warrant Officer. Their technical specialty is "Infantry Weapons Officer", and their role is described in the USMC Infantry Training and Readiness Manual:

The Marine Gunner is a Chief Warrant Officer specifically trained in the employment and training of infantry battalion organic weapons, gear and assigned personnel, and in the Combat Marksmanship continuum. Marine Gunners are special staff officers employed as the principal advisor to commanders at all levels. They assist in the development of training and employment plans designed to ensure Mission Essential Task compliance. ... They mentor the officers and Marines of the unit in all applicable mechanical, doctrinal and conceptual weaponeering and training matters as required in order to improve the general effectiveness and proficiency of the command. They have oversight of the unit's ammunition allocation and annual weapons requalification and certification programs. ... The Gunner's strength is the ability to vet, conduct and quantify combined-arms training and to introduce and sustain basic marksmanship tenets and principles across the Marine Corps.

Fundamentally, they still perform the same role as in 1917. It is seen as the most demanding warrant officer role in the USMC, and the prerequisites include 16 years of service and having reached the rank of Gunnery Sergeant (E-7, where E-1 is a private, E-2 a PFC, etc.), compared to 8 years service and a rank of Sergeant (E-5) for other USMC warrant officers.

You might be interested in this online database of WW2 USMC deaths: https://www.naval-history.net/WW2UScasaaDB-USMCbyNameA.htm

Reference:

USMC Infantry Training and Readiness Manual: https://www.marines.mil/Portals/1/Publications/NAVMC%203500.44C%20Infantry%20T-R%20Manual%20(secured).pdf