Garry's Mod System Requirements and Gameplay Video

Minimum:

OS: Windows® Vista/XP/2000
Processor: 1.7 GHz Processor
Memory: 512MB RAM
Graphics: DirectX® 8.1 level Graphics Card (Requires support for SSE)
Hard Drive: 150 MB
Other Requirements: Internet Connection

Recommended:

Processor: Pentium 4 processor (3.0GHz, or better)
Memory: 1GB RAM
Graphics: DirectX® 9 level Graphics Card