Real World article
(written from a Production point of view)

Referenced once in behind-the-scenes dialogue, the gangorian clap was a type of sexually-transmitted disease.

While filming Star Trek Beyond, McCoy actor Karl Urban ad-libbed a line in which McCoy told James T. Kirk that the Gangorian clap had passed from Kirk to Nurse Chapel, which Kirk professed to know already and which McCoy said Chapel wasn't very happy about. ("Gag Reel", Star Trek Beyond Blu-ray special features)