You can spawn weapons from the "Weapons" tab. By default you will have the Garry's Manhack gun. If you have Half-Life 2: Episode 2, you will also have the Flechette Gun. Counter-Strike: Source guns will show up if you have Counter-Strike: Source.
Note: You can also right-click a weapon to spawn it without picking it up.
Additional scripted weapons, or SWEPs, can be downloaded from http://www.garrysmod.org/.