One of the items being auctioned at Greater Boston PFLAG’s annual fund-raiser Thursday is a Celtics No. 98 jersey signed by Jason Collins, the NBA player who recently acknowledged that he’s gay. In his Sports Illustrated story, Collins revealed that he chose 98 for the C’s (and later the Washington Wizards) because 1998 was the year University of Wyoming student Matthew Shepard was kidnapped and tortured in an attack that cost him his life and galvanized the gay community. “My one small gesture of solidarity” was to wear 98, Collins wrote. Thursday’s event in the Kelleher Rose Garden in the Fens is a benefit for the national nonprofit that is made up of parents, families, and friends of lesbians and gays.