I will discuss how proton decay combined with gravitational waves can be used to test Grand Unified Theories (GUTs). In particular, proton decay searches by largescale 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 that can produce a stochastic background of gravitational waves, which several gravitational wave detectors will be sensitive to over a wide frequency range. I will 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 and their compatibility with leptogenesis as a means of producing the observed matter-antimatter asymmetry. Additionally, I will extend this discussion to include supersymmetric GUTs, taking into account recent findings from Pulsar Timing Arrays that have detected gravitational waves in the nanoHertz frequency range.
