The Gauge is an item acquired in Prehistoria, on the Volcano Slopes. It was once part of the boiler system in the Volcano, and ended up on the slopes after its eruption. Tinker Tinderbox sends the player to retrieve it as part of his plans to build a rocket.
The Gauge is in a pot on the top area of the slopes, reached by riding the geysers to the Volcano crater; the crater is now full of lava and the player will drop past it to land on the ledge with the Gauge. The pot with the Gauge can be seen earlier in the game, but cannot be reached.
Due to a glitch, multiple Gauges can be acquired; once the player turns it over to Tinker, they can return and find the pot filled with another Gauge. If the player does this three times, the three Gauges will count as the needed items for Tinker completing his rocket, letting the player skip getting the Diamond Eyes and Wheel.