A gate is a mini boss from Wario Land: Shake It!. Guarding the Merfle Barrel in each of the three Subwarine levels, it attacks by generating enemies and switching on environmental features in an attempt to defeat Wario.
There are three types of these mini bosses in the game.
Located in Wavy Waters, the first gate attacks by generating submarines and diver Bandineros. To defeat it, Wario must shoot the red button that appears in its core with missiles, which will eventually cause the machine to explode and let Wario finish the level.
Another gate appears in Windbreak Bay. Again located at the end of the stage, this version has three cores that Wario must destroy with missiles, and uses underwater fans to try and push Wario around.
The final gate is located in Creep Blue Sea. Just like the one in Windbreak Bay, the gate here has three cores that Wario must destroy with missiles to progress, but now also has spikes littered around the floor and ceiling.