You don't need GSAP to add a class to an element, just use .classList.add(yourClass);
The problem with that is that it either has the class or it does not, there is no in-between state, so nothing will animate, see below. You're on the GSAP forum, so I think you look to animate things, then just create an animation for each property you want to change, much more fun and easier to control! Hope it helps and happy tweening!
https://codepen.io/mvaneijgen/pen/dyLzJxg?editors=0011