I'm attaching below an ebuild for tkdiff, a simple Tk GUI frontend to diff. Not much more to say about it, and the ebuild is very straight forward. I suggest sys-apps/tkdiff, as this is the location of other diff frontends.
Created attachment 47346 [details] tkdiff-4.0.2.ebuild
Actually tkcvs includes this tkdiff (not sure about the version though), so I guess this is redundant.
Closing this.