Gil's work is to fight monsters and make them tired enough to make it possible to "catch" them into magical scroll so it'll be easier to find new home for those monsters later or keep them safe until the cure from the Monster Virus is found.
Bright color helps to catch Monster's attention so it won't hurt civilians or other colleagues.
Collar keeps a track of Gil's heartbeat and other body changes to make sure if he's okay during the fight.
Backpack is only used during Gil's adventures to other worlds, because his other work is to find "temporary" homes for monsters in worlds without any danger AND without civilization (so Monster won't hurt any civilians once again). This backpack is magical and has a whole room inside of it, where Gil stores needed things or lives in there.