There seem to be two gtk2hs libraries on Hackage - gtk and gtk3. Looking at the source, they seem to share the same files and have only minor differences. How are these libraries different?
As pointed out in the comments, gtk binds to GTK+ 2, and gtk3, to GTK+ 3. One place where that information shows up is in the pkg-config fields of their respective .cabal files (cf. gtk.cabal and gtk3.cabal).