Recent research studies of magnetic topological insulators (TIs) such as MnBi2Te4 have led to the discovery of interesting phases of matter including quantum anomalous Hall insulators (QAHIs) and axion insulators . On the other hand, utilizing magnetic proximity effect on TIs has also succeeded in producing QAHI states , making proximity-coupling another viable way to induce magnetic ordering in topological surface states. We experimentally observed anisotropic magnetoresistance (AMR) in 3D TI/ferrimagnetic insulator devices, Bi2Se3 on yttrium iron garnet (YIG) thin films at temperatures below 10K. The results indicate that the AMR in the exfoliated Bi2Se3 on YIG is related to the out-of-plane magnetic order induced on the TI surface, which rotates with the application of the external magnetic field in different orientations . We further investigate the AMR's dependence on an electric field applied via a top oxide layer. The results reveal how the energy gap is tuned with both magnetic and electric fields.