I'm using a Raspberry Pi Zero for my balloon as well, but it doesn't have a camera on it (yet). You can check out the source on GitHub here (feel free to use it, GNU GPLv3).
The GPS unit I'm using (like most others) output their data via NMEA strings over a UART serial port (reversed so TX->RX and RX->TX). Whenever you parse an NMEA string, make sure to check the "fix" value, since completely incorrect data can be sent over if the fix value is not correct (zero typically means invalid, higher numbers means more accurate).
My GPS code parses air speed in knots, altitude in meters, latitude and longitude in degrees. More data can be retrieved from there, but it's mostly statistics on the GPS network itself.
The camera shouldn't be hard if you know C programming, since you can just write it to the disk. Sending it over and handling it in software would depend on what transmission system you are using.