Sharksie forked the PlayerGui in 2014 at the time and added some extra functionality, and it still works.
For the source code, you can view the abandoned GitHub repository, which was active in 2014. To do this, click on the commit and then click “Browse files”. You will be limited to April 2014 though since that is when the repository was set up.
Commits ending in 2014: Commits · Roblox/Core-Scripts · GitHub
