If you're using a workout tracking system, most of them use gender as a basis for compiling a leaderboard. As a result all workout tracking systems require a gender to onboard your members into their systems.
If you do not enter a gender, your customers will not be able to be automatically pushed into the workout tracking system you use.
Some workout system gate their paywalls around the number of members you have. For systems like these, you might notice after you hit that cap, your members will no longer be auto-pushed once you hit this cap.
Some systems, like SugarWod, only allow a customer to join one gym. If this customer (as identified by email address) is already in the system, SugarWod will not allow them to be re-created.