The Tau player shouldn't optimise his list to play G.K, that would be list tailoring and extremely dirty.
True.
Good against-all lists should be used for the best results.
Any dimwit can build an anti-X list but then what result will it give and what light will it shine on the army if only used against what is it built against?
True skill lies in building a list that has a good chance of faring well against anything it can meet and really interesting results will come from having several of those lists face of against each others.