emerge -pvc x11-libs/gtk+:2 Calculating dependencies... done! x11-libs/gtk+-2.24.32-r1 pulled in by: media-gfx/viewnior-1.7-r1 requires x11-libs/gtk+:2
Last upstream commit was on 2018-11-25. It iss very unlikely that it will be ported to GTK 3+.
Looks like this project still has got some life left in it.