In order to play version 9, I believe you will need HL2 and other Source games. For version 10 (for $10) you just need only one source game so you can use it for any source game you want. I would recommend just to buy GMod10. It will be more convenient.
I first bought CS:Source, then GMOD 10, then HL2: Episode 2, then DOD:Source. You can use GMOD 10 for all of these games and most other source engine games, if not all.
EDIT: Oh SHOOT, I had no idea this was a bumped thread.