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Hi,
yes, there was a bug but it is fixed in the current version.
Please use the latest version.
Or fix it yourself like I did 2 years ago:
http://www.feyncalc.org/cgi-bin/cvsweb.cgi/HighEnergyPhysics/qcd/GluonPropagator.m.diff?r1=1.4&r2=1.5&f=h
rolfm@gluon:~> math5
Mathematica 5.2 for Linux x86 (64 bit)
Copyright 1988-2005 Wolfram Research, Inc.
 -- Motif graphics initialized --
In[1]:= <<HighEnergyPhysics`fc`
In[2]:= $FeynCalcVersion
Out[2]= 5.1.0beta2
In[3]:= GluonPropagator[p,Mu,a,Nu,b,Gauge->{Momentum[n],
alpha},Explicit->True]//FCE//InputForm
Out[3]//InputForm=
I*FAD[p]*SD[a, b]*(-MTD[Mu, Nu] +
 (FV[n, Nu]*FVD[p, Mu] + FV[n, Mu]*FVD[p, Nu])/SP[n, p] -
 (FVD[p, Mu]*FVD[p, Nu]*SP[n, n] - alpha*FV[n, Mu]*FV[n, Nu]*SPD[p, p])/
  SP[n, p]^2)