Their FAQ says kind of:

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. […] 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.

The requirements that GrapheneOS has on the hardware, like relocking the bootloader and hardware level access, should be part of rights to repair / digital markets act imo. They are even considering producing their own hardware in the future.