Yes, the pipework (either horizontal loops or boreholes) are buried at a depth where the ground temperature is consistent all year round, thereby the winter months do not effect the efficiency of ground source heat pumps. This can be viewed as both a positive and a negative thing - for example if you are looking to heat a swimming pool during the summer months then an air source heat pump will be more efficient than a ground source heat pump due to the ambient temperature of the air vs that of the ground.