Maybe.. kind of.

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

They bought the HL1 rights back from Sierra in the early 2000s. The real problem is that the code is not in a distributable state and nobody at Valve feels like working on putting it into a distributable state ( https://github.com/ValveSoftware/halflife/issues/1712#issuec... ):

> A while back Valve [had] a partner perforce server that had depots of the source files for both gold source and source that were shared with development partners and some mod teams. This server had a major meltdown and those depots were lost. At the time there was no requests and no activity around gold source development. Resources to rebuild the depots did not exist and still don't so that code is just not available. Once a year someone talks about maybe pulling it together to open source it but once again there are not resources to do the actual work need to package it up. The Sven Co-op team was luck in that there was a package and someone to make it available to them, that does not exist today.