As Optikon said in his response, there are many possible Gray codes.
The definition of a Gray code is (as Optikon said) that only one bit changes at each step.
A 4 bit Twisted Ring Counter would have the following sequence:-
0000, 1000, 1100, 1110, 1111, 0111, 0011, 0001, 0000, etc.
Gray codes are also used in position sensors. Say you want to convert the angular position of a shaft into a digital signal. You could attach a transparent disc to the shaft with say a 4 bit code on the disc (eg. a photographically produced transparent disc with the ones and zeros represented by transparent and non transparent sectors. There would be 4 LED/Photo diode (or transistor) pairs to detect the data.
If you used binary numbering, it would not be possible to detect the exact code at some transistions since often 2 or more bits change at once.
Whereas, with a Gray code, since only one bit changes at each transition, there is no ambiguity.
Len