I've never been a GAPAN/HCAP member, but may as well point out that it really is one of these "ancient" guilds (more properly called Livery Companies).
There's a list of all 110 of them on Wikipedia:
https://en.wikipedia.org/wiki/Livery_company
On the whole, whilst I tend to think that the criticism of the Masters medal is fair, and yes it is an Establishment Club - it's one that is open to anybody to join and isn't at-all secretive.
I've never joined for the simple reason that you can only belong to so many organisations, and I consider it less useful
to me than for example the Society of Flight Test Engineers. But in general terms, the basic fact and existence of HCAP / GAPAN is entirely reasonable and above board. I've been invited professionally to a few of their specialist meetings, and the quality of what they were doing was excellent.
Two other "establishment" clubs I belong to: the Society of Experimental Test Pilots, and the Royal Aeronautical Society are both very happy to also dress up for dinner - and the first of those is American with a headquarters in California (and boasts the aforementioned Tim Peake and Jim Lovell as members). We're human, we love our ceremony and tradition - it's no reason to criticise anybody. It would be a far poorer world without a bit of pomp and dress-up.
G