https://grapheneos.org/faq

Many other devices are supported by GrapheneOS at a source level, and it can be built for them without modifications to the existing GrapheneOS source tree. Device support repositories for the Android Open Source Project can simply be dropped into the source tree, with at most minor modifications within them to support GrapheneOS. In most cases, substantial work beyond that will be needed to bring the support up to the same standards. For most devices, the hardware and firmware will prevent providing a reasonably secure device, regardless of the work put into device support.

if fair phone wanted to, they could, but gos will not volunteer for the work.