you use hammer in one of the steam tool i forgot what the program is called but on the tools selection its in there then when you start it up hammer is the first choice
Hammer is in Source SDK. Dont know if you can get the vmf file (the one that you can edit in Hammer) of the stock DOD maps, if not you have to find a decompiler to get the bsp file into vmf format.
I know I've got a link to a decompiler DL somewhere, you can probably google it but let me know if you can't find one.
Yup, ALL game content that is not custom is in the .gcf file. It stops people changing the game code without people noticing (i.e. it stops hacks) because there shouldn't (there actually are) be any editor of .gcf files.