In sticky in this forum, you have a post by The-No0n3:
http://www.freesteam.org/forums/showthread.php?t=455
SOURCE GAMES
o Mandatory Files
+ platform.gcf
+ sourceinit.gcf
+ winui.gcf
o Base Files (Source)
+ base source engine 2.gcf
+ Source Shared Securom.gcf (maid not be required)
+ source engine.gcf
+ source materials.gcf
+ source models.gcf
+ source sounds.gcf
o Counter Strike: Source
+ Base Files (Source)
+ counter-strike source shared.gcf
+ counter-strike source client.gcf
I believe you alredy have the mandatory files in your steam even if u didn't install any game yet. As of the other files.. Maybe i'll edit soon with a topic. (But just look in the GCF forum.. there are a few topics)