It’s got GPS functionality, but that’s not secret and is in fact advertised by the manufacturer: LTE frequency bands - #8 by Richard
However, the datasheet (also linked in that post) indicates that the device has connection points for 2x cellular radio antennae, and then another connection point for the GPS antenna. Seeing as how Purism have already specified a completely separate satellite positioning chip for navigation data (Massive Progress, Exact CPU Selected & Minor Shipping Adjustment – Purism), I don’t see them wiring up the GPS antenna from the modem.
That said, GPS frequencies are 1.58 and 1.23 GHz. While cellular radio signals don’t use those exact frequencies, they do cover that range and since you don’t have one antenna for every single frequency from 700 MHz to 3.7 GHz, they’re going to be broadband antennae capable of receiving the whole spectrum - including GPS.
While it would in theory be possible to route the incoming signal from one of the cellular antennae to the GPS receiver, I don’t know how feasible that would be - the amplification and noise filtering requirements are much more stringent for GPS than cellular radio signals, perhaps requiring a different pre-amplifier stage entirely (and ideally, an antenna which is perfectly matched to the source frequency for better signal reception - the received power level is incredibly low, far less than a cellular signal). Can this even be done? I’ve no idea. I don’t know what kind of signal routing Qualcomm (the Gemalto PLS8 uses a Qualcomm modem, but the datasheet doesn’t tell me which one exactly) put into their chips, nor what their pre-amplifier requirements are.