Warning! This takes time to get set up, do us all a favor and don't try to do this while driving. We need you here to discover other clever hacks
Oh yeah, this absolutely sucks battery from your phone. make sure the phone is plugged in.
You'll need to set up two connections between your gTab and data phone: Wireless Tether and Bluetooth GPS.
I'll assume both devices are rooted. I don't know which mod I have on my phone, but on the gtab, I have TnT Lite v2.00. If this works on other mods, please post.
Setup on the Phone:
Install WirelessTether (free from the market) This creates a wireless Ad-Hoc AP that your gTab can connect to for internet access.
Install ShareGPS (free from the market) This outputs the GPS signal over bluetooth your gTab can connect to for GPS data.
Setup on the gTab:
Patch /system/bin/wpa_supplicant - see here for details - http://forum.xda-developers.com/showthread.php?t=754961 This allows the gTab to connect to Ad-Hoc (as opposed to Infrastructure) wireless APs. I personally extracted the file from the zip file in the link and overwrote the one in my gTab using Root Explorer.
Go to Setting->Applications->Development and check "Allow Mock Locations"
Install Bluetooth GPS (free from the market)
Once you've got the apps installed, you can try it out.
Pair your phone and gTab:
Turn on Bluetooth on the phone.
Settings->Wireless & networks->Bluetooth Settings Check Discoverable
On the gTab, Settings->Wireless & networks->Bluetooth Settings Check Scan for devices.
Click on your phone to pair
Click "Pair" on the Pair Request popup on both the phone and gTab.
You only have to do this part once.
Start the Tether:
On the Phone, Start Wireless Tether. Press the screen to start the tether.
Turn on Wi-Fi if it's not on already
On the gTab, go to Settings->Wireless & networks->Wi-Fi Settings
connect to (*)AndroidTether
Start the GPS Sharing:
On the Phone, run ShareGPS - Wait for a GPS fix, you'll see a LAT/LONG on the screen.
On the gTab start Blutooth GPS, Check the box for "Enable Mock GPS Provider", Select your phone and Press Connect
You should now see GPS data in the Main Tab.
Click the Map Tab and see your location on the map! W00t!
Start Google Maps
You know it from here. Navigation works, but I think the mic might not be up to the task for voice search.
Big Thanks to roebeet, and eveyone else for making this possible.