Gas pipes are hazardous obstacles in Pikmin 2. As the name would indicate, they are small tubes that emit a steady stream of poison gas. They can commonly be found in caves by themselves, though some are present in the overworld as well, commonly inside bramble gates or bridges.
As they constantly emit poison, a hazard only White Pikmin (and Bulbmin) can handle, White Pikmin (or Bulbmin) generally must be used to deactivate them, by attacking them like an enemy. Once deactivated, it will stop emitting poison. However, gas pipes can be deactivated with non-White Pikmin as well. For this, one must throw a Pikmin directly onto the pipe. Although uncommon, the Pikmin will sometimes land a hit before starting to choke. From this point, the Pikmin can just be whistled back, and the cycle can repeat until eventually the pipe is deactivated. It's also possible to bait a Gatling Groink into attacking the pipes, which will gradually decrease their health. This can only be done to the gas pipes on the gate after the lake in the Wistful Wild.
On the surface of areas, gas pipes that are placed inside gates or bridges function as a means of only allowing White Pikmin to be used to break down the obstacle. For gates, however, the top two thirds can still be latched-on to by thrown Pikmin, regardless of type. If the player hasn't unlocked White Pikmin yet, it will be impossible to cause damage to the gates. The exception to this is the ones found in the Wistful Wild: without White Pikmin, the gas pipes can still be deactivated with a Gatling Groink, and to be able to destroy the gates afterwards, the player has to end the current day or enter a cave (in order for the gas pipes to disappear).