Using content from other games
You can access any number of games through Steam, even if they don't use Source, by tweaking your gameinfo.txt. This doesn't require extracting anything from a GCF, but you can't guarantee that the user owns the games you've mounted.
Note:If you are thinking of extracting content from a GCF and distributing it yourself, see the next section.
Legally-speaking however, making (for example) a Source mod that features music from Defcon may not be permissible, even though only those who own Defcon could play it. This is because there are two competing rights:
The right for the creator to dictate how their works are distributed
The right of those granted access to works (e.g. by buying them) to mix them together for personal use There is no legal precedent for the quasi-authorized content mixing that Steam allows.Source
Anyone who has uploaded a map .bsp with content embedded in it has done the same thing. Distributing materials that you arnt sure if the person using is authorized to use.