The GND wire is often black in colour. It may be called GROUND or Negative or even B- (battery -). The diagram may just show the the letters GND or show one of the symbols below to identify the GND connection.
The GND wire is part of the ISO standard so colour is 99.99% of the time black. GND may be the thicker of the two black wires if there is more than one. Position A8 of the ISO power plug is often used for GND connection.
Your custom fit android headunit will have the earth (GND) connection already in the loom, but if you are adding an aftermarket backup camera, then you will need to find a suitable GND point for its connection.
Where to connect GND
GND or Ground is the vehicle negative earth. The -ve (negative) terminal on the car battery and nearly every other metal body and chassis part is also connected to the same ground. Vehicle manufactures use this common earth to limit the number of wires required as they will just make a connection to the car chassis to get a GND connection rather than installing more cables. You can reuse a suitable vehicle earth point or even run a new connection to the car battery if a new GND connection is needed.
With every car now ether having a factory installed stereo or at least the optional radio as a pre-installed wiring loom you should be able to connect your android headunit to this connection. If you are unsure which connection is earth (GND) then use a continuity tester to check between the suspected vehicle cable and the metal body of the existing stereo. If there is no stereo fitted, check for continuity between the vehicle loom and an point or the battery negative terminal.