Simply search GitHub directly for repositories, and look for the "Pages" link in the repository description to see if a live demo is available.
Use browser extensions to block ads if you are worried about malicious pop-ups, though GitHub-hosted games are generally cleaner than traditional "unblocked games" websites. Conclusion
Yes, playing games hosted on GitHub Pages is generally safe. Unlike sketchy flash game portals filled with intrusive pop-up ads, malware downloads, and tracking scripts, GitHub Pages sites are static. They do not have the authorization to install software on your computer.