An abbrieviation of GoldSource. It is a modified version of the Quake Engine and was used by ValvE, a seattle based games company, to create the award winning PC title Half-Life. It also served as the engine for Counter-strike and Team Fortress Classic.
There is also a large community that still works with the engine today (despite it being nearly 10 years old), creating maps and mods with custom made content.
NewMapper: "Can anyone tell me how to make a character speak to me in my map?"
VeterenMapper: "Source or Gldsrc?"
VerterenMapper: "NO U"