Strange.
Which Operating System and which Mathematica
Version are you using?
For me it just works:
rolfm@gluon:~> math
Mathematica 5.1 for Linux x86 (64 bit)
Copyright 1988-2004 Wolfram Research, Inc.
-- Motif graphics initialized --
In[1]:= << HighEnergyPhysics`FeynCalc`
In[2]:= $FeynCalcDirectory
Out[2]= /home/rolfm/.Mathematica/Applications/HighEnergyPhysics
In[3]:= $FeynCalcVersion
Out[3]= 5.1.0beta1
In[4]:= Tr[GS[a,b,c,d]]
Out[4]= 4 (a.d b.c - a.c b.d + a.b c.d)
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