LOS ANGELES — The first sign of what is to come at Nerdstrong Gym in North Hollywood is the red welcome mat: It’s in the shape of a Dungeons & Dragons 10-sided die. The equipment inside includes 25-pound steel clubs (for the potions class portion of a Harry Potter workout, among others). A gray concrete wall is painted with pixelated Space Invaders from the 1970s video game. And the gym’s handbook reassures members that workouts can be scaled from Padawan to Jedi Master (that’s Star Wars for “pretty easy” to “really hard”). Since this gym opened in 2013, members —...