Proton decay is a smoking gun signature of Grand Unified Theories (GUTs). Searches by Super-Kamiokande have resulted in stringent limits on the GUT symmetry breaking scale. The large-scale multipurpose neutrino experiments DUNE, Hyper-Kamiokande and JUNO will either discover proton decay or further push the symmetry breaking scale above $10^{16}$ GeV. Another possible observational consequence of GUTs is the formation of a cosmic string network produced during the breaking of the GUT to the Standard Model gauge group. The evolution of such a string network in the expanding Universe produces a stochastic background of gravitational waves which will be tested by a number of gravitational wave detectors over a wide frequency range. We demonstrate the non-trivial complementarity between the observation of proton decay and gravitational waves produced from cosmic strings in determining $SO(10)$ GUT breaking chains. We show that such observations could exclude $SO(10)$ breaking via flipped $SU(5)times U(1)$ or standard $SU(5)$, while breaking via a Pati-Salam intermediate symmetry, or standard $SU(5)times U(1)$, may be favoured if a large separation of energy scales associated with proton decay and cosmic strings is indicated. We note that recent results by the NANOGrav experiment have been interpreted as evidence for cosmic strings at a scale $sim 10^{14}$ GeV. This would strongly point towards the existence of GUTs, with $SO(10)$ being the prime candidate. We show that the combination with already available constraints from proton decay allows to identify preferred symmetry breaking routes to the Standard Model.
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