G-BLASTN is free software and you can browse/download the source code at sourceforge.net or github.
Binary codes of G-BLASTNA are also avialble for the following platforms:
- 64bit CentOS 6.4: https://github.com/OpenHero/gblastn/releases/download/v1.1/gblastn
- 64bit Windows 7: http://sourceforge.net/projects/gblastn/files/1.1/bin/gblastn.exe/download