CartesianToLorentz[exp] rewrites Cartesian tensors in form of Lorentz tensors (when possible). Using options one can specify which types of tensors should be converted.
CGS[p]
% // CartesianToLorentz\overline{\gamma }\cdot \overline{p}
p^0 \bar{\gamma }^0-\bar{\gamma }\cdot \overline{p}
CSP[p, q]
% // CartesianToLorentz\overline{p}\cdot \overline{q}
p^0 q^0-\overline{p}\cdot \overline{q}