CGSD[p] is transformed into DiracGamma[CartesianMomentum[p, D-1], D] by FeynCalcInternal.
CGSD[p,q, ...] is equivalent to CGSD[p].CGSD[q].
CGSD[p]\gamma \cdot p
CGSD[p] // FCI // StandardForm
(*DiracGamma[CartesianMomentum[p, -1 + D], D]*)CGSD[p, q, r, s](\gamma \cdot p).(\gamma \cdot q).(\gamma \cdot r).(\gamma \cdot s)
CGSD[p, q, r, s] // StandardForm
(*CGSD[p] . CGSD[q] . CGSD[r] . CGSD[s]*)CGSD[q] . (CGSD[p] + m) . CGSD[q](\gamma \cdot q).(m+\gamma \cdot p).(\gamma \cdot q)