It isn't possible to provide GrapheneOS in the form of a GSI. GrapheneOS has core privacy and security protections requiring kernel and driver changes. The project you linked doesn't include the kernel hardening, USB protection, hardened_malloc, MTE, various userspace features depending on kernel changes, verified boot and other unlisted features. Various other features were disabled by default. Many features only partially work. GrapheneOS is based on AOSP 17 but this isn't yet.