Currently, gnome-base/gvfs is pulled in via the gnome or gnome-lite meta ebuild. However, this doesn't work when just using bits and pieces of gnome, such as evolution under xfce4. The gnome-base/libgnome ebuild has a direct dep on gnome-vfs, but no direct dep on gvfs, which is meant to replace it. Please add one :) Without gvfs, gnome-open from gnome-base/libgnome does not work for opening URLs.
BTW, both 2.24.1 in the tree and 2.26.0 in the gnome overlay need this additional dep.
Fixed in overlay. Thanks for reporting.
and also in tree.