If there is a rubber seal / o-ring and the connector doesn't leak after the nut is tightened, than there is no extra sealability added with PTFE tape over the thread. It is no good for swivel connectors.. Next to that, the standard PTFE tape has a max pressure resistance 16 bar, without any additive to make it high pressure resistant it might just blow out at the 50 bar from the CO² bottle.
Anyway, CO² as gas, you can easily check for leakage with a bit of dishwashing soap in water. Spray this on the joint, if it leaks you'll see soap bubbles appear. If it does and its sufficiently tightened, than replace the o-ring with a new one.
