There are lot of improvements and fixes since last stable release. There is much improved support for Samsung, Siemens and Motorola phones, new features in SMSD (message filtering, injecting for FILES mode) or support for OpenCellID.
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.