From GMod Wiki
Garry's Mod comes with one weapon in this menu, The Manhack Gun. Other weapons become available when you mount other games:
Half Life 2: Episode 2
- Flechette Gun
- Desert Eagle
- Pump Shotgun
Using Scripted Weapons
- Left-clicking any weapon will arm the player with that weapon.
- Right-clicking any weapon will spawn it where the cross-hair is aimed in-game. Walking over the spawned weapon will arm you with it, or give ammunition if you already have it.
- If a server that you are playing on does not have the weapon you are trying to spawn, clicking it's spawn icon will not spawn it.