Loadout comes from the 1980s and aviation. An aircrafts 'loadout' was its weaponry for a certain mission profile.
A strike aircraft would have a 'loadout' of say MK 82s and rockets pods... so its slowly crept into meaning any type of equipment used for a certain areana, i.e. A 'Marine loadout' would be all the needed equipment to 'be' suited for purpose... i.e. pass as a Marine.
Translation: Some really gay sounding term that airsofters use to describe their kit or gear that they spend all of their Birthday and paper route money on.