The best open source alternative to Garry's Mod is s&box. If that doesn't suit you, our users have ranked more than 25 alternatives to Garry's Mod and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Garry's Mod are Novetus, Manic Digger, Golems Universal Constructor and World of Padman.
S&box is a platform and game development toolkit developed by Facepunch studio powered by Source 2. Initially, the development was based on Unreal Engine 4, but as soon as Half-Life: Alyx came out, Facepunch immediately requested the engine branch of the newly-published game.
Tired of the roblox tyranny and greed? The Novetus community game is the solution! Novetus was made to improve on RBXLegacy, with the addition of new features that enhance the launcher for mod development, player customization, and overall usability.
this is an alternative to roblox, not garry's mod
Manic Digger is an open-source implementation of block-building games, like the famous game Minecraft.
ITS JUST A FREACKING COPY OF MINECRAFT!
Golems Universal Constructor or Golems is a 3D recreational physics simulator.
Build robots, spaceships, clocks, catapults or cannons.
Add sensors, timers, switches, motors. Give it a brain. Make a mouse trap - or make a mouse.
Take the wheel and pilot your machine yourself.
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WoP is the domain of ENTE's comic strip super-hero bad-boy Padman and his motley crew, and their mission is to make your game with some seriously addictive fun, whatever your skill level and whatever character role you jump into.
United Kingdom
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Newman should of made it open sourced.