FCTraceFactor[expr] factors out all expressions inside a trace to which the trace doesn’t apply. For example, all objects that are not Dirac matrices can be safely factored out from every Dirac trace.
Overview, DiracTrace, SUNTrace.
Pull constants out of the Dirac trace
FCTraceFactor[DiracTrace[c1 . (c2*(GS[p1] + M)) . GA[\[Mu]] . (c3*(GS[p2] + M2))]]\text{c1} \;\text{c2} \;\text{c3} \;\text{tr}\left(\left(\bar{\gamma }\cdot \overline{\text{p1}}+M\right).\bar{\gamma }^{\mu }.\left(\bar{\gamma }\cdot \overline{\text{p2}}+\text{M2}\right)\right)