I think you may have just misunderstood this episode. Ben wasn't in the process of releasing the gas. The people from the boat knew that Ben had the ability to release the gas and kill everyone on the island (as he did before), so they wanted to eliminate that possibility. Ben sent Juliet to stop them. When she arrived at the Tempest, the redhead and Faraday were in the process of trying to render the gas inert (i.e., make it no longer dangerous). That process apparently caused the pressure in the gas tanks to rise, almost causing their inadvertent release. Which is why they were wearing the hazmat suits, just in case.