With the Taranis, I got a GPS using the “SmartPort” interface from FrSky. It's basically a Serial bus on which you can add many sensors, which are (randomly ?) polled by the master of the bus.
Luckily the Teensy3 can do that. If you don't want to mess with voltage conversion, just use a Teensy3.1.
Most of the information comes from the OpenTX firmware, information on the Internet and direct analysis of the communication bus, so it might not be perfectly accurate.
I don't have anything permanent, it's all on breadboard for now, so there's no schematic. Just connect the signal pin of the sensor to the pin 1 of the Teensy (TX1, PB17), Gnd to Gnd and… you're done. You can power the GPS using the Vin pin of the Teensy, it makes development easier.
Here's the last code: tsysportmaster_20140916.tar.gz
I found different projects interfacing the Teensy3.1 with a SmartPort receiver (like the X8R):