Which 2022 Portal spin-off was released as free DLC for owners of Portal?

The 2022 Portal spin-off released as free DLC for Portal owners was Portal with RTX.

Developed by NVIDIA Lightspeed Studios, Portal with RTX is a ray-traced reimagining of Valve’s original 2007 puzzle game. It uses NVIDIA’s RTX Remix technology to rebuild lighting, reflections, materials, and other visual effects while preserving Portal’s familiar test chambers and puzzles.

The DLC launched on December 8, 2022, and was offered free to owners of Portal on Steam. It was not an expansion for Portal 2, a common mix-up caused by the similar titles and the fact that several Portal 2 projects are also free mods or add-ons.

Portal Stories: Mel and Portal Reloaded are community-made projects associated with Portal 2, while Aperture Tag is a separate Portal 2-based fan game. None is the official 2022 RTX release.