Gates are green doors found in many chapters (levels) of the game. They appear in both versions, but have different purposes.
Wii version Edit
In the Wii version, gates are linked to Legacy Medals. In each level that has Legacy Medals, 3 of the medals (1 Bronze, 1 Silver, and 1 Gold) are each hidden in a bonus 2D room (which also contains Gravitons).
The main level has 3 gates, each leading to one of the 2D rooms. These gates have a number above them, indicating the number of Gravitons required to go through the gate; if Rodea doesn't have enough Gravitons, the gate won't open.
In each 2D room, a second gate will lead Rodea back to the main level. Each gate disappears after being crossed.
Wii U/3DS version Edit
In the Wii U/3DS version, a gate (called "door") can be found in some levels. When crossed, it leads to a remote island with 3 dash rings: one leads to mech pieces, another one to Gravitons, and the last one to either nothing or enemies. The dash rings look identical, so it is not possible to know which one leads to what beforehand. However, the same gate always leads to the same island. After taking a dash ring and potentially getting items, Rodea will be taken to another gate that leads back to the main level.
Each gate disappears after being crossed.