How do I mount other Source games to GMod and HL2 and Episodes Hammer editors?

[Bruner]

At some point I want to try and get into making maps in the Source engine for ether Garry's Mod or Half-Life 2: Episode 2. However, I need to know how I can use content from other Source games such as HL2 Episode 1, HL2 Episode 2, and CS:Source. Perhaps if possible using assets from the 2003 leak. Help would be very appreciated!