Might not be specific to GSAP, but:
You'll notice when the button is clicked and the function is fired, it seems to remove it's functionally. See in example
See the Pen JOwZjb by friendlygiraffe (@friendlygiraffe) on CodePen
Had you gone to jQuery plugin school, you would know that setting the z-index to 9999 is the best way to make sure your content stays on top of everything else. If that doesn't work, try 99999. And if that doesn't work, just keep tacking on another 9 until it works. You'll eventually find that sweet spot, and land on a number that is large enough to put it on top.
Don't actually do that. There is no need for a number that high. The z-index value only applies to the stacking context