There's a pool service in Arizona, Nevada, Texas and southern California called Pool Chlor and they use mostly chlorine gas without the problems you mention (as far as I know -- do you know which pool service it was that had the problems you have seen?), but they test the chemistry and dose known amounts. Their pools have high CYA of around 100 ppm which helps keep the chlorine from breaking down as quickly in the intense hot desert sun. They typically dose up to around 14 ppm FC and a week later the FC has dropped to around 4 ppm so this is sufficient to prevent algae growth and is a range of active chlorine that doesn't irritate. 14 ppm FC with 100 ppm CYA has the same active chlorine concentration as 4 ppm FC with 30 ppm CYA. As for pH control, they just have a higher Total Alkalinity (TA) level, usually around 120-140 ppm. Even with 120 ppm TA and 100 ppm CYA, going from 4 ppm FC to 14 ppm FC using chlorine gas lowers the pH from 7.5 to 7.2 or from 7.7 to 7.34. The pH then rises slowly over the week as carbon dioxide is outgassed (TA is a mostly a measure of the over-carbonation of the water). In 15,000 gallons, one would need to add about 5 cups (about 3 pounds) of baking soda to maintain the TA level over the week (the TA would drop by about 15 ppm over a week due to the combination of outgassing, which raises pH, and chlorine usage, which is acidic and lowers pH and TA). So using chlorine gas can work very effectively IF you know what you are doing. As with anything else, if you don't know what you are doing, serious problems can occur.
