What Gigastructure is
The official locations article describes Gigastructure as a gravity-warped, shifting labyrinth of stairs and corridors.
Update additions
The Gigastructure update introduced Alpha Flowerman, Tentacle, Alpha Mimic and the Vanilla Mall Nexus map, alongside documented behavior changes for several creatures.
Exact floor layouts, timers and rewards are not treated as fixed facts without current-client evidence.