Can we include some variable(e.g. x,y,z,...) in values={}? For example,

{a,b,c,d}::Indices(values={0,1,2, 3, z},position=fixed);
\Gamma{#}::GammaMatrix(metric=\eta);
\eta*{a b}::KroneckerDelta;
C*{a b c}::AntiSymmetric;
D*{a b c}::AntiSymmetric;
ex1:= C*{a b c} \Gamma^{a b c};
ex2:= C*{a b c} D*{a b c};
rl:= { C*{0 1 2} = \alpha, C*{1 2 3} = \beta, C*{1 2 z} = \gamma, D*{1 2 3} = \theta, D_{1 2 z} = \tau};
evaluate(ex1, rl);
evaluate(ex2, rl);

from which we would like to expect

\alpha \Gamma^{012} + \beta \Gamma^{123} + \gamma \Gamma^{12z} \beta \theta + \gamma \tau,

respectively.