Ionic and redox control of magnetism can produce large changes to a variety of magnetic properties using a relatively small voltage. A model structure that continues to be of interest is Pt/Co/GdO _xH _y/Au, where the Co magnetic layer oxidation state and perpendicular magnetic anisotropy can be toggled using voltage control of proton transport through the GdO _xH _y electrolyte layer. The hydration of the oxide layer to form a hydroxide phase is the key to improve the speed of these magneto-ionic devices, but there is insufficient understanding of protonic defect incorporation and transport during hydration and electrical gating. In this work, we use polarized neutron reflectometry (PNR) to observe the effects of hydration and electrical gating by scanning in an as-grown state, a hydrated state, and in operando during electrical gating. We directly measure the depth profile of hydrogen and confirm the transformation from oxide (Gd₂O₃) to hydroxide [Gd(OH)₃]. We observe the accumulation of H in the Co magnetic layer and the effects of gating on the structure and hydrogen content of the other layers in the device stack. Using PNR and secondary ion mass spectrometry, we find evidence for much more complex chemistry at the Co/GdO _xH _y interface than was previously assumed, including evidence for persistent CoO phases and CoO _xH _y phases. We offer insight on using PNR to observe relatively fast proton dynamics in the system and fitting a rather complex set of parameters to achieve a physical result for the fit spectrum and scattering length density profiles.