GI1 Android 16 itself is open source. It's the Pixel support which is less open source than it used to be before Android 16. It's still more open source than other Android device support. It's part of why we're more interested in focusing on other devices in the future with official OEM support for GrapheneOS.
Lucario1829 Security preview releases are made by applying additional source code patches on top of the regular releases. The code for it is available to GrapheneOS and will be open sourced when the embargo ends. The current security preview releases cover the November 2025, December 2025 and January 2026 patches but January 2026 is currently empty so all of the security preview releases up to this point will have those patch sources released in December 2025.