ExplicitLorentzIndex[ind] is an explicit Lorentz index, i.e., ind is an integer.
ExplicitLorentzIndex[ind]
ind
Overview, LorentzIndex, Pair.
Pair[LorentzIndex[1], LorentzIndex[\[Mu]]]
gˉ1μ\bar{g}^{1\mu }gˉ1μ
Pair[LorentzIndex[1], LorentzIndex[\[Mu]]] // StandardForm (*Pair[ExplicitLorentzIndex[1], LorentzIndex[\[Mu]]]*)