This paper examines the effect of GNSS spoofing on GNSS-IMU fusion-based position, velocity and attitude estimates. First, it proposes and tunes an estimator considering also IMU sensor biases and real flight data of a multicopter. Then it feeds the estimator with IMU data from a fixed flight trajectory and GNSS data from different trajectories with increasing divergence from the fixed one simulating a perfect spoofing scenario. Detailed examination of the estimates shows that spoofing has non negligible effect on velocity and especially attitude estimates. Thus any spoofing detection algorithm can not be based on attitude estimates which utilize GNSS data.