According to the wiki, GoldenEye: Source requires "At least one Source Based game purchased and installed." I'm not 100% sure if this is true, since I don't know if the mod shares any files from CSS, HL2, or any of the episodes, but if it is ture, GoldenEye: Source (Along with any "open" source mod) just became more open to the public.
The most common graphic cards are either made by ATI or Nvidia, and both card types them have free Source games available for them. For those who have an
ATI card, they can go
here to get HL2: Lost Coast and HL2: Deathmatch. The others who have a
Nvidia card can go
here to get HL2: Loast Coast. HL2: Deathmatch, Portal: The First Slice (Which is now free), and Peggle Extreme.
~IN THEORY~, you could use the free demo "Portal: The First Slice" as your source game for those who don't have either an ATI or Nvidia card, but I have never tried since I bought HL2 when it first came out.
Hopefully this can help those who want to play this, but don't own any Source games. If this has been brought up before, I'm sorry, but it would be a good idea to add to the wiki so people know about it.