The 2421, the excess‐3 and the 84-2-1 codes are examples of self‐complementing codes. Such codes have the property that the 9's complement of a decimal number is obtained directly by changing 1's to 0's and 0's to 1's (i.e., by complementing each bit in the pattern).

Is GREY code self complementing?

They are not self complementing, as I can see.

Is 3321 a self complementing code?

codes whereas 5211,2421,3321, 4321 are self complementing.