CGA[i] can be used as input for \gamma^i in 4 dimensions, where i is a Cartesian index, and is transformed into DiracGamma[CartesianIndex[i]] by FeynCalcInternal.
Overview, GA, DiracGamma.
CGA[i]\overline{\gamma }^i
CGA[i, j] - CGA[j, i]\overline{\gamma }^i.\overline{\gamma }^j-\overline{\gamma }^j.\overline{\gamma }^i
StandardForm[FCI[CGA[i]]]
(*DiracGamma[CartesianIndex[i]]*)CGA[i, j, k, l]\overline{\gamma }^i.\overline{\gamma }^j.\overline{\gamma }^k.\overline{\gamma }^l
StandardForm[CGA[i, j, k, l]]
(*CGA[i] . CGA[j] . CGA[k] . CGA[l]*)DiracSimplify[DiracTrace[CGA[i, j, k, l]]]4 \bar{\delta }^{il} \bar{\delta }^{jk}-4 \bar{\delta }^{ik} \bar{\delta }^{jl}+4 \bar{\delta }^{ij} \bar{\delta }^{kl}
CGA[i] . (CGS[p] + m) . CGA[j]\overline{\gamma }^i.\left(\overline{\gamma }\cdot \overline{p}+m\right).\overline{\gamma }^j