On the SpaceFM website (http://ignorantguru.github.io/spacefm/) you can see that it depends on "GTK+ v2 or v3 (2.18 thru 3.x supported)". At the moment the spacefm-0.9.0 ebuild contains a dependency on GTK 3.x only. It also contains the --with-gtk3=yes configure option. With a couple of small edits I managed to obtain an ebuild that works with GTK 2.x as I prefer to avoid version 3 for the moment. I think we could add a gtk3 USE flag for those who prefer the latest version.
see bug 420493 as for why I will not do this