You can add GCP with XYZ using precise XY and guessed Z taking it from google earth, for example, then set accuracy accordingly, for example an accuracy of 0.005/10 would mean 5mm in horizontal positioning and 10 meters in vertical.
Also, if you are using a drone geo-tagged images, you can use 10/10/0.2 for camera accuacy. So the drone altitude measuered by the baro altimeter (which is often quite accurate in relative) will make your model to be properly leveled while GCP will put it into right XY
Regards