Welcome to the Greensock forum.
You'd want to get the progress() when the tween/timeline updates and that will be a value between 0 and 1. You'd then use that to feed into your percentage counter. I did something quite similar in this demo:
Lines 10 and 11 are the timelines that call the changeIt() function onUpdate. On line 30 you'll see how the progress() updates the text. This demo is SVG, but works the same way for regular text.
Here's some more info about progress()
If you have other questions, a CodePen demo makes it easier to get you the best answers. Here's some more info about that.
Hopefully that helps.
Happy tweening and welcome aboard.