For barre class, you want a good mix of form-fitting and loose-fitting garments so that you can add or remove layers depending on your temperature. Keep things ballet-inspired with soft pinks, white, and black. The only accessory you will need is a pair of grip socks and I recommend the kind with crossing straps to maximize the ballerina aesthetic.
The key to dressing for spin class is choosing garments that are moisture wicking since you will basically produce your body weight in sweat. It's best to choose a capri bottom as shorts leave your legs exposed to chafing and long leggings can sometimes restrict your range of motion. As for the top, the best combination is a great sports bra underneath a loose-fitting tank top. After class, you'll want to have your favorite facial mist on hand for a refresher.
If you're trying to emulate Gigi Hadid, you are likely going to sign up for boxing lessons. Start your workout with a sweatshirt to warm up your muscles and then remove once you start to sweat. Keeping things minimal is best, so try a matching set sports bra and leggings. If you want to go all out and look ridiculously cool, buy a pair of these gold boxing gloves from Fashercise.