The GMI takes into consideration the number of an armed forces' heavy weapons in relation to the total population. Heavy weapons are defined here as any piece of military equipment which fits into either one of four categories: armored vehicles (armored personnel carriers, light tanks, main battle tanks), artillery (multiple rocket launchers, self-propelled artillery, towed artillery) above 100mm caliber, combat aircraft (attack helicopters, fixed-wing fighter aircraft), and major fighting ships (submarines, major surface combatants above corvette size).

Data on weapons holdings was collected by BICC from different sources, mainly the Military Balance from ISS. Data on small arms and light weapons (SALW) is not only extremely difficult to obtain but also unreliable and was thus not included in the GMI.