GS[p] can be used as input for a 4-dimensional p^\mu \gamma_\mu and is transformed into DiracGamma[Momentum[p]] by FeynCalcInternal (=FCI).
GS[p,q, ...] is a short form for GS[p].GS[q].
Overview, DiracGamma, GA, GAD.
GS[p]\bar{\gamma }\cdot \overline{p}
GS[p] // FCI // StandardForm
(*DiracGamma[Momentum[p]]*)GS[p, q, r, s]\left(\bar{\gamma }\cdot \overline{p}\right).\left(\bar{\gamma }\cdot \overline{q}\right).\left(\bar{\gamma }\cdot \overline{r}\right).\left(\bar{\gamma }\cdot \overline{s}\right)
GS[p, q, r, s] // StandardForm
(*GS[p] . GS[q] . GS[r] . GS[s]*)GS[q] . (GS[p] + m) . GS[q]\left(\bar{\gamma }\cdot \overline{q}\right).\left(\bar{\gamma }\cdot \overline{p}+m\right).\left(\bar{\gamma }\cdot \overline{q}\right)