Russell,
Yes, these are hydrogen gas bubbles. They are not chlorine gas since this gets dissolved almost immediately into the water to become chlorine in water (hypochlorous acid). This is nothing to worry about, except these bubbles do provide an excellent source of aeration which is why the pH tends to rise in pools with an SWG. Keeping the TA low (about 80 if the CYA is 60-80) and the pH target higher (about 7.7) will reduce the tendency of the pH to rise and significantly reduce the amount of acid needed to keep the pH down.
Richard
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