Van der Waals (vdW) heterostructures allow the engineering of magnetic and electronic properties via interfacial orbital hybridisation and charge transfer. The resulting electric fields can give rise to Rashba spin-orbit coupling and Dzyaloshinskii-Moriya interactions, which may stabilise novel two-dimensional magnetic phases. While such emergent magnetic 'interphases' may offer a promising platform for spin-based electronics, direct spectroscopic evidence for them is still lacking. Here, we report electric-field tunable Zeeman effects with Landé g-factors up to ≈230 at the interface of graphene and the vdW ferromagnet Fe₃GeTe₂. They arise from an interplay of local-moment canting and the itinerant orbital moments of Fe₃GeTe₂, producing a giant asymmetric level splitting. Exploiting the inelastic phonon gap of graphene, scanning tunnelling spectroscopy allows us to access the buried interface properties, and quantify Faraday-like screening. Our findings provide new insights and opportunities for electrically controlled interface magnetism, e.g. for spintronics and orbitronics devices.