You need the jcal (libjalali-dev in ubuntu) in order to build this program. If you have installed jcal in somewhere else than /usr you will need to change /usr/lib/libjalali.la in gahshomar/src/c/Makefile.am accordingly.

Once you have jcal installed, download the source code from github or download the latest release from the releases page and type:

./autogen.sh --prefix=/usr
make
sudo make install

If you are using Arch Linux, you can install gahshonar using the aur package.