Garry's Mod minimum requirements for Windows assume having at least Windows XP/Vista operating system. Your processor should be 2 GHz or a more powerful one. The minimum RAM requirement is 4 GB. As for a graphics card, it should be 512MB dedicated VRAM, and it should support Direct X Version 9.0c. You also have to have 5 GB free disk space to install the game.
If you go for recommended requirements for Garry's Mod for Windows, your operating system should be Windows 7/8/8.1/10. Recommended CPU would be 2.5 GHz CPU or equivalent. For RAM, it is recommended to have 8 GB or more. And video card — 1GB dedicated VRAM. It should also support Direct X Version 9.0c.
As for Mac OS, Garry's Mod requirements here start with OS X version Snow Leopard 10.6.3 operating system. Processor needs to be at least 2 GHz. 4 GB of RAM is required. Your graphics card should be 512MB dedicated VRAM. Finally, the game needs 5 GB of free disk space.
As for recommended OS, it is Latest OS X version. 2.5 GHz processor is recommended. Plus, 8 GB of RAM. Your video card should be 1GB dedicated VRAM.
If you’re on Linux, minimum requirements include Ubuntu 12.04. Processor should be 2 GHz or higher. For RAM you need 4 GB or more. Graphics card should be 512MB dedicated VRAM or better. 5 GB of free disk space is a must.
For Linux recommended requirements, use Latest Ubuntu. As for the processor/CPU, use 2.5 GHz or equivalent. Add 8 GB or more of RAM to it. And make sure you have 1GB dedicated VRAM graphics card.
Garry's Mod is a physics sandbox. There aren't any predefined aims or goals. We give you the tools and leave you to play.