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The control over electrical conductivity is critical key to enablingultrawide bandgap materials, e.g., for high power electronic devices. Identifying defects and their electronic properties remains a challenge. Here, we introduce frequency-domain Terahertz Electron ParamagneticResonance (EPR) ellipsometry as a new fully polarization-resolving spectroscopy tool to study defects in semiconductors thin films and heterostructures at very high magnetic fields and very high frequencies. We determine the full spectral and magnetic polarization response of EPR signals as a continuous function of both field and frequency. THz EPR ellipsometry is shown previously for the nitrogen defect in SiC. Here, we investigate Fe-doped gallium oxide single crystals, and detect rich spin signatures which strongly vary with crystal orientation, frequency, and field. The neutral defect Fe3+ is a high-spin system with s=5/2 and large zero-field splitting. Iron canincorporate at either Ga site but appears preferentially in octahedral configuration. Different claims exist about the nature of the spin Hamiltonian and approximate values for simplified orthorhombicmodels have been reported. We obtain the anisotropic g-factor as well as the zero-field Hamiltonian up to fourth order which allows to discuss the relevance of the monoclinic character of the local site symmetry.