Maybe 'gill' isn't the best wording. To be more precise: the diving bell spider's air chamber's wall behaves like a membrane, allowing oxygen to diffuse inside and carbon dioxide to diffuse out of the air chamber. However, depending on the oxygen level inside the water this phenomenom can only expand the time the spider can spend underwater, if the oxygen level is too low it still has to return to the surface eventually and gather new oxygen supplies.
Kill me now.