I think what Tom is saying is that fish gasping is not a good co2 indicator , because you can have a very low co2 level and fish gasping because of low o2 , and you can have a good co2 without fish gasping if you have good surface ripple and good o2... ...if i understood that right..
My understanding is that CO2 won't displace O2. So if you have a low o2 level, you have a low o2 level, irrespective of whether your injecting CO2 or not. In other words, if you turn the CO2 on, and the fish head for the surface, then turn the CO2 off and the fish are ok again, only one variable is changing - the CO2 level - not the O2 level. I cannot see how fish gasping as a direct result of changing the single CO2 variable is not a good indicator of good CO2 level, ethics aside. This is of course assuming that there isn't a pre-existing O2 problem (i.e. tank very understocked with fish etc).