The NPC emailed me back. All they wrote was "Thank you for reaching out to the National Panhellenic Conference. Please visit the NPC website for some history information here:
https://www.npcwomen.org/resources/p...Friendship.pdf.
Also, based on our historical review we could not find information regarding Beta Gamma. Theta Upsilon affiliated with Delta Zeta in 1962. Iota Alpha Pi became an associated member of NPC in 1955, then became an active member in 1957 but withdrew in 1977 due to diminished membership."
So it seems that Beta Gamma was a weird mistake. Maybe the wiki page editor meant a chapter of a sorority named Beta Gamma?