Most modern receivers are transmitters also in the sense that they emit electromagnetic radiation at radio frequencies. In the case of the GPS receiver ther are several conversions from very high frequencies and processing of the signal. This involves using oscillators that produce internal frequencies that are in parts of the radio spectrum and modulated in ways that can interfere with some communications and other systems succeptible to RF interference. These signals are not particularly powerful, but can cause problems.
Sorry for getting off topic.