CGSE[p] is transformed into DiracGamma[CartesianMomentum[p, D-4], D-4] by FeynCalcInternal.
CGSE[p,q, ...] is equivalent to CGSE[p].CGSE[q].
CGSE[p]\hat{\gamma }\cdot \hat{p}
CGSE[p] // FCI // StandardForm
(*DiracGamma[CartesianMomentum[p, -4 + D], -4 + D]*)CGSE[p, q, r, s]\left(\hat{\gamma }\cdot \hat{p}\right).\left(\hat{\gamma }\cdot \hat{q}\right).\left(\hat{\gamma }\cdot \hat{r}\right).\left(\hat{\gamma }\cdot \hat{s}\right)
CGSE[p, q, r, s] // StandardForm
(*CGSE[p] . CGSE[q] . CGSE[r] . CGSE[s]*)CGSE[q] . (CGSE[p] + m) . CGSE[q]\left(\hat{\gamma }\cdot \hat{q}\right).\left(m+\hat{\gamma }\cdot \hat{p}\right).\left(\hat{\gamma }\cdot \hat{q}\right)