Plays inside your own copy of Garry's Mod, on your device, not in a browser tab. Opens github.com in a new tab.
How to play
Have your own copy of Garry's Mod installed: the mashup plays inside it.
Download it from its release page on GitHub (it runs on Windows, macOS and Linux).
Install it as its page says. What it asks for is just below, in the list's words.
Start the game the way its page says: some mashups come with a launcher of their own.
What it asks for
Garry's Mod on the x86-64 beta branch, Minecraft Java with an account, Prism Launcher and the bundled launcher; optional dedicated GMod and Minecraft servers for multiplayer. Linux x86-64 is primary per the README.