The Doom source code was originally released under a non-commercial license that was weirdly restrictive and it was eventually re-released under GPL. The Quake source code was released under GPL from the beginning.
If Valve really wanted to release HL1/GoldSrc source code, they could re-base to the GPL quake source code and release their changes as GPL as well. This would be a miserable job because the remaining quake code is probably scattered across the codebase in weird orphaned fragments, but afaik it would be completely legal.
e: oh yeah if the shambling zombie that is Sierra still holds any rights over HL1 then god knows what the IP situation is with that property
The Doom source code was originally released under a non-commercial license that was weirdly restrictive and it was eventually re-released under GPL. The Quake source code was released under GPL from the beginning.
If Valve really wanted to release HL1/GoldSrc source code, they could re-base to the GPL quake source code and release their changes as GPL as well. This would be a miserable job because the remaining quake code is probably scattered across the codebase in weird orphaned fragments, but afaik it would be completely legal.
e: oh yeah if the shambling zombie that is Sierra still holds any rights over HL1 then god knows what the IP situation is with that property