Lottie handles a different usecase. Lottie is used to port animations from after effects to the web as an svg animation. For this Lottie will use a static json file which is generated beforehand. It uses a quite complex structure and is mere impossible to read as a human. Making dynamic manipulations not feasible.

For dynamic animations gasp is great as you just code the animation in js making it a perfect fit for the web. The creation process can be quite cumbersome though.