This version changes the goal from “Steve + a few blocks” to a Minecraft-style survival sandbox running inside Garry's Mod.
Garry's Mod is the primary host. Minecraft: Java Edition is the companion: the addon is designed to read the player's Minecraft installation at runtime and use its installed resources rather than redistribute Minecraft's files.
SOURCE / NOT YET TESTED IN GMod. This environment does not expose a running GMod installation, so I cannot honestly claim a gameplay test.
Copy gamemode/addons/minecraft_in_gmod to garrysmod/addons/minecraft_in_gmod.
Run tools/preflight.py before packaging. The Python importer is deliberately separate from the addon so the source archive does not contain Minecraft assets.