it's a perl app, and most of the cpan dependencies can be installed via synaptic. the one requirement that is causing an issue is the Sane CPAN package. the version in the repos is 0.03 and 0.05 is needed.
tried downloading and compiling it directly from cpan, but ran into issues. i don't have any scanner backends installed which is why i think it's dying.
you can always try to download this and install:http://search.cpan.org/dist/Sane/
expand the .tar.gz file. run: perl Makefile.PL, then make and if there are no errors, as root make install
gscan2pdf should compile and install then