In the design field we call this "back of house."
It includes:
night quarters/barns for animals
off-exhibit holding
possibly quarantine areas for sick or newly introduced animals
chutes etc. to move animals from barn to exhibit to off-exhibit holding, etc
keeper locker room, "break" space, offices
animal food storage areas
Commissary (central location for animal food storage)
loading docks, etc
veterinary facilities
zoo administrative offices (director, Curators, Human Resources,Accounts, Fund Raising, etc.)
Meeting Rooms
food service storage, offices, loading docks, etc
gift store offices, storage, etc.
Horticulture Department offices, equipment storage, perhaps plant nursery and greenhouse
Maintenance Dept. offices, equipment storage, materials storage
a gas station to refill zoo vehicles
lots of vehicles (but never enough)
lots of roads connecting all these things
there's more, but you get the idea
The first 6 items on my list are the ones most immediately behind that gate