Harness, belay device, and shoes. That's all you need to get started climbing at the gym. You may not even need the harness or belay device, if you're just starting out bouldering. And climbing in the gym sure beats burning miles on the treadmill, especially in bad weather.
Every climber loves a few regular trips to the gym. It's more social, it's a great environment for testing your limits, and it's the perfect way to elevate your dynamic moves and your strength-to-weight for any sport. And it's the perfect place to introduce a friend to the stoke of pushing themselves in a new sport. Nothing bonds people together like sharing new adventures. So there's a good reason gym climbing is skyrocketing in popularity: it's a fun way to get out & get away from screens, have fun with a partner -- no matter the weather, and no matter how far from the crag you may live.