CSS3 transitions have some significant limitations that make them unworkable for a serious animation platform. They don’t provide precise controls over the timing or easing. They’re great for simple effects but the GreenSock Animation Platform delivers extremely precise rendering, so you can do things like pause() and reverse() an animation anytime or skip to a specific time and play from there, etc. Try creating a CSS3 transition that uses an elastic.out or slow motion ease and then jump to 0.72494-seconds into a 2-second transition and pause() only to resume() later. It’s impossible from what I understand. So no, the platform doesn’t make use of CSS3 transitions. However, it is highly optimized for performance. See the detailed cage match where GSAP battles CSS3 transitions where there’s a detailed comparison in several categories.

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.

IntertiaPlugin is a membership benefit of Club GreenSock, so you need to sign up to get that. Draggable, however, is included in all the standard downloads and you don't need a membership to get that; Draggable can be used apart from InertiaPlugin as long as you don't need the momentum-based motion. GreenSock's standard license allows you to use GreenSock tools for free in everything except a very specific type of commercial project (if you collect a fee from multiple customers for the app/product/game/site that uses GreenSock tools) which makes it extremely accessible and business-friendly while providing a small funding mechanism to sustain ongoing support, enhancement, and innovation. The web is littered with abandoned “open source” projects, but GreenSock has a years-long track record of commitment to the platform. This unique licensing model is a key component of its sustainability. If you charge multiple customers a usage/access/license fee, please simply sign up for a “Business Green” Club GreenSock membership which comes with a special commercial license granting you permission to do so. Click here for details. Joining the club also gets you members-only bonus plugins, utilities, update notifications, and more.