If it's big enough you could just put the showtime stage area from buydebug on top of it, if I recall it's 7x8, then put the seating squares in front for the audience to stand/sit in, the stage makes the game spawn the proprietor placement after a while, I made a basement in the Water Hole Juice bar in Riverview and put in a stage, the lot was set as a local watering hole at the time, it worked perfectly, had simfests and regular shows, it's possible it would work if left as a gym but I can't say for certain as they did add the function for showtime stages in bars.