The correct solution for now is to build from darcs. Detailed instructions are given in the Getting the latest and greatest section of the installation instructions; the short version is:
darcs get --lazy http://code.haskell.org/gtk2hs
cd gtk2hs
sh bootstrap.sh
EDIT: The official 0.12.5 release of gtk2hs from December 2013 supports cabal 1.18, so the above darcs instructions are no longer needed. The full installation instructions are still available from the Gtk2Hs download page; the short version is:
cabal install gtk2hs-buildtools
cabal install gtk gtk3