alley cat – just like the electrical sub-board inside your Lot, the gas sub-meter that’s installed inside (the kitchen cupboard) of that Lot would have been installed there as a compliance condition at the time of your building’s construction, and as such it’s downstream of the prime meter and your responsibility.
So I think that your Strata Manager’s advice is correct, but for confirmation try contacting the gas supplier as opposed to the retailer who sends your accounts; so far as I’m aware that’s the successor to part of the former Australian Gas Light Company that now goes by the name of Jemena in NSW.