CGA fittings are parallel threads. For any seal with parallel threads, some type of washer is necessary. A permaseal will work, but a cheap nylon seal will work just as well. Any shop that sells or refills CO2 will have them.
Most of the threads we deal with are NPT, which are tapered threads. For those, you use some type of sealant (teflon tape, pipe dope, loctite) for a seal.