Yes, GrapheneOS only works on Pixel devices, because the project has some pretty extensive hardware security requirements: https://grapheneos.org/faq#future-devices

The Fairphone is a highly insecure device, which comes nowhere close to the (hardware) security of a Pixel. On top of that, the Fairphone company doesn't even know how to maintain their own Fairphone OS. The verified boot implementation is fundamentally broken and very misleading, since it's signed with the publicly available (!!!) AOSP test private keys. This is such a blatant disregard of security practices, that should have made it impossible to certify their devices. It's not a surprise either that Fairphone regularly misses important Android security patches, or delivers them months later. That's also why GrapheneOS will never support devices like the Fairphone. There are more issues with Fairphone's misleading update policy that I haven't covered in detail.

I highly recommend against purchasing such insecure, and poorly maintained hardware. DivestOS is the best option for "damage control", if you already own a Fairphone. Its developer actually cares about users and their security, and the OS is properly signed.