GMod is a Sandbox Modification. Using GMod you can pose ragdolls. This means can position people from HL2/CS/DoD and change their faces. A big comic book creating community has popped up around this feature. Using GMod you can build. Take props from HL2/CS/DoD and weld them together to make walls. Axis a wheel to it to create a working car. Rope some cans to the back. A big contraption creating community has popped up around GMod - creating such things as Rube Goldberg devices, catapults, cannons, bridges etc.. Using GMod you can make your own weapons. That's right, make your own weapons. Using Lua you can create a custom weapon to do just about anything. If you're new you can change simple things like rate of fire, models, sounds. If you're awesome the world is your oyster, mass changing guns, melon cannons, jetpacks. GMod gives you the freedom to do any of these things in single player or online with your friends.
Finally, Day of Defeat: Source shootable weapons for garrys mod.
I decided to put DoD:S weapons in garrys mod. I did NOT create the models, Valve did. These are real, shootable guns, not just props. To install, put in C:/localdisc/ programfiles/ steam/ steamapps/ yoursteamusername/ garrysmod/ garrysmod/ addons/ day-of-defeat/ lua. If you do not have the day of defeat directory in your addons, you can make it. You NEED Day of Defeat: Source in order for this to work. Please report any errors or things you would like to change. This is the beta version. If this does not work, please let me know because I know what the problem is. Enjoy!