> the critical sensor to me is GPS

How accurately do you care about geolocating yourself?

If you only care about <50m accuracy for the watch to be able to provide you location-based behaviors, I've had very good results doing Wi-Fi scans with ESP32 devices (in particular, a FeatherS2) and feeding the results into the Google and Mozilla geolocation APIs. More than enough to pull up the local weather or public transit info for the bus stop in front of you. It could send data requests through your phone when you don't have Wi-Fi.

Obviously it won't have the accuracy and update rate that a GPS would, but on the upside, it does work indoors.