Monolayer Cr2Ge2Te6 (ML-CGT) has attracted broad interest due to its novel electronic and magnetic properties. However, there are still controversies on the origin of its intrinsic magnetism. Here, by exploring the electronic and magnetic properties of ML-CGT, we find that the magnetic shape anisotropy (MSA) is vital for establishing the long-range ferromagnetism, except for the contribution from magnetocrystalline anisotropy energy (MCA). Electronic band analysis, combined with atomic- and orbital-resolved magnetic anisotropy from a second-order perturbation theory, further reveals that the MCA of ML-CGT is mainly originated from hybridized Te-py and -pz orbitals. The MSA from magnetic Cr atoms in ML-CGT is larger than MCA, resulting in an in-plane magnetic anisotropy. Noticeably, by constructing a heterostructure (HTS) with ferroelectric Sc2CO2, CGT undergoes an in-plane to out-of-plane spin reorientation via ferroelectric polarization switching, accompanied with an electronic property transition from semiconductor to half-metal. The Curie temperature of CGT/Sc2CO2 HTS can be enhanced to 92.4 K under the ferroelectric polarization, which is much higher than that of pristine ML-CGT (34.7 K). These results not only clarify the contradiction of magnetic mechanism of ML-CGT in previous experimental and theoretical works, but also open the door for realizing nonvolatile magnetic memory devices based on a multifunctional ferromagnetic/ferroelectric HTS.