GSD[p] can be used as input for a D-dimensional p^\mu \gamma_\mu and is transformed into DiracGamma[Momentum[p,D],D] by FeynCalcInternal (=FCI).
GSD[p,q, ...] is a short form for GSD[p].GSD[q].
Overview, DiracGamma, GA, GAD.
GSD[p]\gamma \cdot p
GSD[p] // FCI // StandardForm
(*DiracGamma[Momentum[p, D], D]*)GSD[p, q, r, s](\gamma \cdot p).(\gamma \cdot q).(\gamma \cdot r).(\gamma \cdot s)
GSD[p, q, r, s] // StandardForm
(*GSD[p] . GSD[q] . GSD[r] . GSD[s]*)GSD[q] . (GSD[p] + m) . GSD[q](\gamma \cdot q).(m+\gamma \cdot p).(\gamma \cdot q)