$LeviCivitaSign is a global variable that determines the sign in the result of a Dirac trace of four gamma matrices and \gamma^5. $LeviCivitaSign is by default set to -1 which corresponds to the convention TR[LC[a,b,c,d,5]] = -4*I*Eps[a,b,c,d]. Setting $LeviCivitaSign=-I will switch to the FORM-convention.
Overview, LC, Eps, DiracTrace.
$LeviCivitaSign
DiracTrace[GA[\[Mu], \[Nu], \[Rho], \[Sigma], 5]] // DiracSimplify-1
-4 i \bar{\epsilon }^{\mu \nu \rho \sigma }
This sets the same convention as in FORM
$LeviCivitaSign = -I;
DiracTrace[GA[\[Mu], \[Nu], \[Rho], \[Sigma], 5]] // DiracSimplify4 \bar{\epsilon }^{\mu \nu \rho \sigma }
Back to the standard value
$LeviCivitaSign = -1;