Compared to the NX-800, the NX-820 has:
smaller LCD display
less zones/channels
Last I checked, no remote head kit, but that might have changed.
DB15 accessory connector (NX-800 has DB25), so less remote capabilities.
On the up side, the NX-820 has, as you noticed, the built in GPS receiver (still requires an external antenna) and zone/channel ID announcements.
The NX-800 and the NX-820 use different programming software, so if you already have your own software for the 800, you'd need to purchase new software for the 820.
The cost of a GPS receiver is very low. I've purchased small units for $20 or less. The cost savings on the radio is mostly a marketing thing.