Yet another bugfix release, still quite far from calling this stable. The major change here is renaming Port configuration directive to Device as it is easily understand (the old option will still work for several years).
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.
You are downloading Gammu installer for Windows. There is 32-bit version, 64-bit version and 32-bit version compiled with MSVC 9.0 to be compatible with Python 2.7.
You might need to install Visual C++ Redistributable Packages to run Gammu.