You are downloading Gammu source code. To compile it you need some development environment for your platform and CMake tool to configure the sources. There are also some optional requirements for additional features which are listed in INSTALL file in sources. This file also contains more detailed instructions about compiling Gammu.
This source tarball contains Gammu command line utility, libGammu library and Gammu SMSD.
Released on May 20, 2015, see detailed information.
Should you be interested in git access or git snapshots, you can find them on development page.