I practiced Gnosis for two years without having a group at the time, and was having many experiences in the internal worlds. Therefore, what matters is your practice, not attendance to a specific physical Gnostic organization.
As for evaluating the qualifications of a teacher, we can go into meditation and the internal worlds. I personally dedicated to this purpose when seeking to understand (and meet) the Master Samael Aun Weor, until the time came when he concurred to my call and we made a pact by shaking hands, whereby I would receive his wisdom and help, and that I would consign myself as his disciple. Truly, through internal meditation and inner experience is how we follow Christ within, and cease to look for "masters" in this physical plane!
However, we can learn much from a physical instructor, but you must be judicious, because I have known terrible missionaries, exceptionally cruel and cold, many whom who, while speaking about the teaching, do not practice the teaching at all.
"By their fruits you will know them."