You can have an extra HL2 copy if (in my case) a friend gifted me his extra copy of HL2, and then I upgraded my account to Source Premiere which is pretty much every official Valve game out there. Thus I already had HL2 so I know have an extra copy to give out.
Can't really play much with HL2. It's single player only. Gifting the game doesn't give you access to the Source games (DoD, CS, etc) if I understand it correctly.
However you could install Svencoop into your Steam account as that is a free mod and I think Svencoop should work without having to have Source.