General Relativity and Quantum Mechanics are known to be fundamentally incompatible. One approach to aid in the construction of a complete unified model is to allow new observations to point the theory in the right direction. The XGI aids in this search by developing new tests of General Relativity when gravity is highly dynamical and nonlinear, like for example when black holes merge. These tests verify some of the founding pillars of Einstein's theory, such as the principle of equivalence, parity and time-reversal invariance, the polarization content of gravitational waves, and the spin and mass of the graviton. Any observational sign of a violation of these foundational principles would be catastrophic for Einstein's theory and may point us in the direction of a more complete theory.