Are there any more grey codes?
There will be, I haven't looked at them for years but IIRC there are versions with a lot more codes than you have there, a grey code after all is just a code where only a single bit changes for every new value.
Grey codes are normally used for absolute encoders I thought, a relative encoder should have no need for such things.
No - Gray codes are used for relative encoders, Allows you to get your information in 2 bits and keep track of position in software. Absolute encoders have either a whole bunch of wires or use some serial/parallel protocol to give position.
I have seen 2, 3, & 4 bit grey codes. 2 bit is most common, with a Z bit thrown in that goes true for exactly one sequence of the code in every revolution of the shaft.