GPS requires an onboard chip, so other than Marcum making some sort of dongle that connects via the USB port (which they could easily do) a software update is not possible.
I agree with phone GPS (That’s what I use) but I suspect onboard GPS/mapping is really the only major void they have left with the platform…it should also be very easy for them to incorporate as well as others (Humminbird/Lowrance) have been including it for years…