It would seem - going by configure.ac, ldd output and revdep-rebuild test - that x11-libs/libXt is used neither by gtk+ 2 nor 3.
+ 08 Sep 2013; Gilles Dartiguelongue <eva@gentoo.org> gtk+-2.24.16.ebuild, + gtk+-2.24.17.ebuild, -gtk+-2.24.19.ebuild, gtk+-2.24.20.ebuild, + gtk+-3.4.4.ebuild, gtk+-3.6.3-r2.ebuild, -gtk+-3.8.2.ebuild, + gtk+-3.8.4.ebuild: + Drop libXt dependency, bug #483256. Drop old revisions. + You are perfectly right.