Top 4 garrysmod-addon open source projects

Metrostroi is an addon for Garry's Mod which puts you behind the controls of the real and fully simulated subway trains.

Recreation & expansion of NPCs, entities, and weapons from the Half-Life series into Garry's Mod!

Cross-platform .NET Module/Plugin platform for Garry's Mod powered by .NET.

An addon for Garry's mod that contains bunch of bases to make many different types of addons.