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.
Released on mar. 2, 2015, see detailed information.
You can not find your platform here? Choose the correct one on complete listing.
Should you be interested in git access or git snapshots, you can find them on development page.