  We present a minimal Grand Unified Theory model, based on $SU(5)$ gauge
symmetry and a global $U(1)$ Peccei-Quinn symmetry, that predicts the existence
of an ultralight axion dark matter within a narrow mass range of
$m_a\in[0.1,\,4.7]\,$neV. This mass window is determined through an interplay
between gauge coupling unification constraints, partial proton decay lifetime
limits, and the need to reproduce the experimentally observed fermion mass
spectrum. The entire parameter space of the proposed model will be probed
through a synergy between several low-energy experiments that look for proton
decay (Hyper-Kamiokande), axion dark matter through axion-photon coupling
(ABRACADABRA and DMRadio-GUT) and nucleon electric dipole moments (CASPEr
Electric).
