gitk fails with /usr/bin/gitk: line 3: exec: wish: not found when tk >= 8.5 is not installed. The line in the ebuild should not be: tk? ( dev-lang/tk ) but: tk? ( >=dev-lang/tk-8.5 ) wish is a new part of tk and included since 8.5. The error is easily fixed with installing the new tk version.
Wish has been in TK longer than I've been programming computers. # equery f dev-lang/tk [ Searching for packages matching dev-lang/tk... ] * Contents of dev-lang/tk-8.4.19-r1: /usr /usr/bin /usr/bin/wish -> wish8.4 /usr/bin/wish8.4 gitk works just fine with wish from TK-8.4 here. Please include more of your emerge --info and error output.
I am sorry, I remerged tk-8.4 and wish is now where it should be. (It definitely wasn't there before!). Also don't trust Ubuntu user forums about tk etc. ;) Sorry for the trouble caused.