You can spawn various scripted entities from the "Entities" tab. Scripted entites, or SENTs, are objects coded in Lua.
By default, you will only have the Bouncy Ball entity. This is a good example to look at if you want to code your own.
If you have Half-Life 2: Episode 2, you will also have the "Magnusson Device" entity.
You can download more scripted entities from http://www.garrysmod.org/.