These packages make use of gtk3 useflag which indicates that there are issues of migration to gtk+:3.
Gtk3 and similar useflags should be avoided.
Adding to tracker. Please have a look.
+*gretl-1.9.9 (10 Jun 2012)
+ 10 Jun 2012; Justin Lecher <email@example.com> gretl-1.9.7.ebuild,
+ gretl-1.9.8.ebuild, +gretl-1.9.9.ebuild, metadata.xml:
+ Version Bump, set gtk version fix (<1.9.9 = gtk2, >=1.9.9 = gtk3), #420559