This Frama-C Magnesium-20151002, compilation environment is Ubuntu 16.04 (Xenial).
Ocamlc src/plugins/gui/debug_manager.cmi
Ocamlc src/plugins/gui/help_manager.cmi
Ocamlc src/plugins/gui/property_navigator.cmi
Ocamlc src/plugins/gui/gui_parameters.cmo
Ocamlc src/plugins/gui/gtk_helper.cmo
File "src/plugins/gui/gtk_helper.ml", line 1318, characters 9-58:
Error: Unbound module Dgraph
share/Makefile.generic:77: recipe for target 'src/plugins/gui/gtk_helper.cmo' failed
make: *** [src/plugins/gui/gtk_helper.cmo] Error 2
There are two debian (hence ubuntu) packages that allow you to compile programs that link against ocamlgraph: libocamlgraph-ocaml-dev and libocamlgraph-viewer-ocaml-dev, the latter containing GUI stuff, that include the Dgraph module. While the text-based frama-c need only the former, you'll need both to compile the GUI frama-c-gui.