The xdialog 2.2.1 configure script has a --with-gtk2 flag, but gtk1 is used as standard, so it has to be enforced. ebuild should either depend on gtk2 and enforce --with-gtk2 flag or check if gtk2 is installed, i don't know what's the current policy with gtk1 & gtk2 apps and the gtk use-flag
GTK 2 is preffered - changed in 2.2.1-r1.