InsideDiracTrace is an option of DiracSimplify and some other functions dealing with Dirac algebra. If set to True, the function assumes to operate inside a Dirac trace, i.e., products of an odd number of Dirac matrices are discarded. For more details, see the documentation for DiracSimplify.
DiracSimplify[GA[\[Mu], \[Nu], \[Rho]]]\bar{\gamma }^{\mu }.\bar{\gamma }^{\nu }.\bar{\gamma }^{\rho }
A trace of an 3 Dirac matrices vanishes:
DiracSimplify[GA[\[Mu], \[Nu], \[Rho]], InsideDiracTrace -> True]0