Do I need to worry about garabage collection in the JavaScript version of GSAP? What happens when a tween finishes?

No need to worry. Tweens and timelines are automatically made eligible for garbage collection (gc) when appropriate (typically when they finish but if you maintain a reference to an instance so that you can restart it later, for example, it won’t be gc’d out from under you). Basically the system manages gc for you and generally cleans up after itself.