Hi Christian,
> There used to be a package called gamalg for manipulating with
> Dirac gamma matrices (Clifford algebra) in Maxima.
I'm pretty sure gamalg was cut out because it was not
released under a GPL-compatible license.
I think you'll find the relevant discussion in the mailing list
archives from a few years ago. (http://maxima.sf.net/maximalist.html)
HTH
Robert Dodier