The perpendicular magnetic anisotropy (PMA) of metal/ferromagnet (FM)/oxide trilayers is known to depend on the degree of oxidation of the FM/oxide interface. Among the different methods to tune the PMA, magnetoionics is emerging as a promising technique with potential applications in low-power spintronic devices. In this work, the PMA of Pt/Co/AlOx/HfO2 capacitorlike devices was gradually tuned by electric field gating. Hard-x-ray photoelectron spectroscopy (HAXPES) measurements at a synchrotron radiation source, guaranteeing tunable photon energies, a collimated beam, and a large photon flux, have allowed us to probe the composition of the cobalt ultrathin film buried below the dielectric layer of the capacitors, and its evolution upon the application of the gate voltage. The Co 2p HAXPES spectra of the gated devices were compared to those obtained for Pt/Co/AlOx reference samples, for which the PMA was controlled by tuning the Co oxidation with oxygen plasma. For similar magnetic anisotropy states, the two types of samples exhibit equivalent Co 2p HAXPES spectra, with the same weight of metallic Co and CoO signatures. These results constitute direct experimental proof that, in our integrated devices, the gate voltage modifies the PMA through the modification of the oxidation state of the buried cobalt layer driven by oxygen-ion migration.