The lab's list of projects: http://harc.ycr.org/project/
I've seen an early alpha version of GP during the Scratch conference of 2016 (the 2017 conference is wrapping up tomorrow), and it worked awesomely, like a supercharged version of Scratch. A fun thing about it is that it features a very small VM, and the rest of the interface and blocks are written in GP itself, and can be readily inspected and modified.
Also, Alan Kay had a Quora answer on this:
https://www.quora.com/What-was-it-like-to-be-at-Xerox-PARC-w...