Route
Sub-Tree is the only location in GCO Any% where a mix is performed.
The boss fight for Sub-Tree is Fatty Whale, which is the first of two RNG-heavy points in the early game.
This section of the run is pretty much the "reset hell" zone. Top players will reset here often.
Room 1
The first room is a long hallway with a door at the end. Just run forward and hold up to enter the door as soon as possible.