Multiple experiments (17O NMR, 1H NMR, and EPR) have been performed in the past to understand the microscopic parameters that control the magnetic relaxation rate enhancement induced by paramagnetic molecules on neighboring water protons, the so-called relaxivity. The generally accepted theories of the electron spin relaxation of S = 7/2 ions such as Gd3+ (Solomon-Bloembergen-Morgan or simplified Hudson-Lewis) are unsatisfactory for a simultaneous analysis. Recently, an improved theory, where the electron spin relaxation is due to the combination of a static (thus explicitly linked to the molecular structure) and a dynamic zero field splitting, has been developed and tested on experimental EPR data. The model has also been extended beyond the electronic Redfield limit using Monte Carlo simulations. Using the aqua ion [Gd(H2O)8]3+ as a test case, we present here the first simultaneous analysis of 17O NMR, 1H NMR, and EPR relaxation data using this rigorous approach of the electron spin relaxation. We discuss the physical meaning of the calculated parameters. The consequences on future experiments are also considered, especially regarding the analysis of nuclear magnetic relaxation dispersion (NMRD) profiles in the study of Gd3+ complexes.