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Ultrathin 3d-4f synthetic ferrimagnets with perpendicular magnetic anisotropy (PMA) exhibit a range of intriguing magnetic phenomena, including all-optical switching of magnetization (AOS), fast current-induced domain wall motion (CIDWM), and the potential to act as orbital-to-spin angular momentum converters. For spintronic applications involving these materials, the Curie temperature is a crucial factor in determining not only the threshold energy for AOS, but also the material's resistance to temperature rise during CIDWM. However, the relationship between the Curie temperature, the thicknesses of the individual layers, and the specifics of the growth process remains an open question. In this work, we thoroughly investigate the Curie temperature of one of the archetype synthetic ferrimagnets with PMA, the Pt/Co/Gd trilayer, grown by DC magnetron sputtering and characterized with MOKE and SQUID. We provide an interpretation of the experiments we designed to address these outstanding questions through modeling of the deposition process and the induced magnetization at the Co/Gd interface. Our findings demonstrate that the Curie temperature and, by extension, the conditions for PMA and magnetic compensation, of these ultrathin 3d-4f synthetic ferrimagnets are not only impacted by the interface quality, which can be influenced by the sputtering process, but also to a significant extent by finite-size effects in the 4f-material. This work offers new methods and understanding to predict and manipulate the critical temperature and magnetostatic properties of 3d-4f synthetic ferrimagnets for spintronic applications and magneto-photonic integration.