We present angle-dependent measurements of the effective g factor g in a Ge-Si core-shell nanowire quantum dot. g is found to be maximum when the magnetic field is pointing perpendicularly to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces g significantly. g is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field.