Hey Mario and welcome to the GreenSock forums.   That site is indeed made with GSAP. It was made with an old version of GSAP but many of the effects on it could be made much more easily using the new ScrollTrigger GSAP plugin  I think you're going to love the plugin!   Smooth scrolling (also called momentum scrolling) is possible using the ScrollTrigger plugin as Blake shows in this pen. 
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