GoldSrc is the Half-Life 1 Engine, which is a heavily modded Quake 1 Engine (with some added Quake 2 elements.) It is basically the precursor to the Source engine.
As far as i know, there is no stand-alone SDK for GoldSrc, you need to create a mod for a standalone GoldSrc game to do that (like Wkay said, CS 1.6, DoD or HL1)