Given a binary word relation $τ$ onto A * and a finite language X $\subseteq$ A * , a $τ$-Gray cycle over X consists in a permutation w [i] 0$\le$i$\le$|X|--1 of X such that each word w [i] is an image under $τ$ of the previous word w [i--1]. We define the complexity measure $λ$A,$τ$ (n), equal to the largest cardinality of a language X having words of length at most n, and st. some $τ$-Gray cycle over X exists. The present paper is concerned with $τ$ = $σ$ k , the so-called k-character substitution, st. (u, v) $\in$ $σ$ k holds if, and only if, the Hamming distance of u and v is k. We present loopless (resp., constant amortized time) algorithms for computing specific maximum length $σ$ k-Gray cycles.