Long article (that questions whether the gas even does anything meaningful):
The British group, which included four former special forces members, took a different approach.
About 10 weeks before the expedition, the men began sleeping in hypoxic tents, which lower oxygen levels in the air and gradually acclimatized the hikers to conditions on Mount Everest, [the organizer] said.
While hypoxic tents have been used by some climbers for years, the big innovation for the British expedition came two weeks before the excursion, when the men flew to Limburg, Germany, outside Frankfurt, where a doctor ... had been experimenting with inhaled gases in his clinic.
The men wore masks hooked up to ventilators as an anesthesiologist slowly introduced higher levels of xenon into their systems.