Large American cities during the industrial age had amateur baseball leagues made up of blue-collar workers who created their own little societies unseen by the general populace. Men formed their teams representing factories, power plants, and other facilities developed rivalries, and had playoffs and championship series just like the pros. Some of them, in fact, went on to play professionally. For nearly three decades, San Francisco has had a largely unknown tour de force of amateur sports in the San Francisco Gay Basketball Association and its Castro League. Like most organized amateur leagues, its composed of dedicated people who take the sport seriously and fight hard to win. To be fair, though, it does include some levity. Looking at the team names will tell you that right away: the All-City Ballers, Frisco Force, the Backdoor Pass, the Guardians of the Hole. Step into this world this evening at Sunday Open Gyms. Stuart Leung, the associations acting commissioner, says 20 to 35 people turn out for this practice session each Sunday. Anyone is welcome (gay, straight, bi, or other), and its a chance to socialize, exercise, play ball, and see where it takes you. Leung says people of all experience levels show up, and games are conducted on two courts. Some participants have never played, while others played in high school or college, and a few have done time in the pros. Leung says most people who play in the Castro League got their start at a Sunday open session. Leung says to bring a light-colored as well as a dark T-shirt, so when teams are chosen youll be able to tell the good guys from the bad guys.