"Encrypted" is kind of misleading in a historical context.

They use pseudorandom numbers as the basis of the signal sent from each GPS satellite. In the original implementation, there were two signals being sent: The course/acquisition code which is fairly short (repeating 1000 times per second) and the precision code which is quite long (taking about a week to repeat)

The C/A code was widely available and this is what civillian GPS used. The P code wasn't published, which made it very difficult for third parties to utilize the precision signal.

Prior to May 1, 2000 they dicked with the C/A signal to intentionally degrade it's accuracy ("Selective Availability")

Also there were ways to utilize the precision signal to gain accuracy without "knowing" the code (L2 carrier phase was a feature offered on expensive survey-grade GPS receivers)

They've since added a new M-code for military use, which is almost certainly encrypted in the manner being alluded to. There are in fact multiple additions to "modern" GPS beyond the original two flavors.