Ferrimagnetic insulators with perpendicular magnetic anisotropy exhibit unique properties that offer potential for advanced spintronic devices. One of the challenges in exploiting these materials in spintronics lies in the efficient electrical detection of their magnetization. Herein we report the magnetoresistive detection of perpendicular magnetization in insulating ferrimagnetic garnets, enhanced through proximity coupling with an adjacent ultrathin ferromagnetic layer. We design and characterize a hybrid spin-valve device consisting of terbium iron garnet (Tb⁢IG)/Co/Cu/Tb-Co multilayers where Tb⁢IG and Tb-Co are perpendicularly magnetized soft and hard layers, respectively, while Co is in-plane magnetized. We reveal through anomalous Hall effect and x-ray magnetic circular dichroism measurements that the interfacial exchange coupling between Tb⁢IG and Co induces a perpendicular magnetization component in the Co layer, which increases the magnetoresistance readout signal by more than 2 orders of magnitude—reaching 0.2%—compared with the reference system without the Co layer. A notable but less pronounced enhancement is observed in the YIG/Co-Fe-B/Cu/Tb-Co spin-valve device compared with the stack without the Co-Fe-B insertion, demonstrating the applicability of the approach to similar structures. Finally, we show that the magnetoresistive output signal can be effectively used to probe the multidomain state of Tb⁢IG, opening a path for future insulating spintronic device applications.