Gary has become a carpentry teacher at the community centre, after the set he built in previous episode for the production of "Butterfly Lovers" in All The World's A Stage (2000) caught the attention and leading to William recommending him for the job.
"Madame Butterfly", which William mentioned that he likes and not the music group Boney M. to Shirley, it is actually an opera based on the short story of the same name from John Luther Long, written by Giacomo Puccini.
Tammy saying that Shirley went off with William, it would confirm what happened after the production of "Butterfly Lovers" where they were seen with each other in All The World's A Stage (2000) and David looked on awkwardly.