GS[p]+a
is right.
There are basically no explicit matrices in FeynCalc.
GS[p] (or DiracSlash[p] etc.) are algebraic objects.
See also:
http://www.feyncalc.org/FeynCalcBook/Tr/
Regards,
Rolf Mertig
GluonVision GmbH, Berlin
http://www.gluonvision.com
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