Given the accuracy of even cheap quartz oscillators and the limited energy budget of a wristwatch, I would suppose that the GPS receiver is only turned on for a short interval every few hours to synchronise the internal timekeeper. After all, a wristwatch is likely to be in several places (besides an airliner cockpit) where GPS reception is lacking, such as inside buildings or underground, as well as in street canyons, trains and buses where reception can be poor or intermittent.
On a related note, when flying (as SLF!) I often try to get a fix on my smartphone GPS, if only because I don't usually see those numbers on my TomTom. It is actually quite difficult to do so, and requires holding the phone close to the window for several minutes but also that there are a number of satellites at a low altitude on your side of the aircraft. After getting a fix by holding the phone close to the window, it usually manages to retain the fix even if placed say on the aisle seat table. Also it seems to be easier to get a fix on a Boeing than on an Airbus, but I believe there's another thread for that
