Matt Haag wasn’t exactly dressing to impress during his teenage years on the competitive Call of Duty circuit. The 26-year-old recalled, sheepishly, the basketball shorts and Payless kicks he wore on flights to tournaments around the country back in 2008. All of his gamer friends and teammates looked the same way: schlubby.

But in the last decade, something changed. “I just wanted to look better,” Mr. Haag said. “I needed a girlfriend. I was trying to be cooler. It was as simple as that.”

Today, Mr. Haag, known as Nadeshot, is the chief executive of 100 Thieves, an esports organization that makes apparel for competitive gamers and their fans. He has more than 1 million followers on Instagram and routinely uses the platform to model the latest in 100 Thieves merchandise: splashy graphic tees, athleisure hoodies and starchy camo pullovers, which pair nicely with Mr. Haag’s vast collection of Creamsicle-colored sneakers.

The price point for 100 Thieves merchandise compares to many midpriced athleisure brands; shoppers can expect to pay $85 for a hoodie. Other gamers are spending much more on their looks. “The professional players have probably spent $500 to $1,000 on what they’re wearing,” Mr. Haag said. “They’ve got Gucci and Balenciaga. It’s absolutely ridiculous.”