The package builds fine without the build dependency libgtk2.0-dev. Please just drop it with your next upload.
- Bug#967580: libgsf: depends on deprecated GTK 2 Bastian Germann
- Bug#967580: libgsf: depends on deprecated GTK 2 Simon McVittie
- Bug#967580: libgsf: depends on deprecated GTK 2 Simon McVittie