Keep in mind that with GSAP the CSSPlugin is what animates CSS properties within TweenMax. TweenMax includes the CSSPlugin and other GSAP plugins.
But say you weren't going to use TweenMax, but TweenLite, TimelineLite or TimelineMax. Then you would need the CSSPlugin as a separate script since the CSSPlugin is what GSAP uses to animate CSS properties.
Includes all of the following : TweenLite, TweenMax, TimelineLite, TimelineMax, CSSPlugin, EasePack, RoundPropsPlugin, BezierPlugin, AttrPlugin, DirectionalRotationPlugin
Includes the following : TimelineLite, TweenLite, and EasePack. It does not include the CSSPlugin.
So if you use TweenMax in your project then it will already have the CSSPlugin included, you don't have to include it separately.
Hope this helps, Happy Tweening