To earn it, you must return to the Throne of Thunder raid. You can queue for the Forgotten Depths section of ToT's LFR if you can't convince your guild to clear to Tortos. Once your LFR group has defeated Ji-Kun, you can return to the Tortos area by clicking on a feather and flying into the red portal above. Make sure you don't aggro any leftover trash on the way back!
The geysers in ToT have a hidden minigame. Standing on top of one gives you a stacking debuff called Spray Water. Continue riding the geysers until the debuff stacks to 250, which takes both practice and patience. There's no achievement for this, so look for the "new mail" icon instead. You'll receive a message in your mailbox from Taoshi of the Shado-Pan congratulating you on your "ninja-like geyser-jumping abilities." She includes the geyser gun as a reward. Now you can douse friend and foe alike in lukewarm water that smells strongly of Tortos.
If you want to be a really awesome person, you can earn the gun for several friends at once by allowing them to ride on your mount while you deal with those fickle geysers. Good luck!