GSAP is a great library, but there’s a fairly strong Elm cultural bent to not just wrapping existing JavaScript libraries, and not just because the elm community wants to be different. The type safety of Elm is dependent on any underlying JavaScript code being safely minimized, contained and extensively reviewed.
You can, however, interface with JavaScript libraries through ports (which you can learn about in the Elm tutorial I believe). There are also some good animation libraries written in pure elm that will give you both great type safety and idiomatic elm API’s.