New testing release for Gammu is here. It brings some cleanup to SMSD tables, changes library soname and adds dozens of fixes.
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 binaries for Windows. The files are available as installer (.exe) or just an archive (.zip). The -Minimal variant has slighly less features compiled in and contains Gammu.dll, which you might use in external programs. There are also python-gammu binaries, which include all needed parts to use python-gammu in your python programs, just choose the version matching Python version you use.
You might need to install Visual C++ Redistributable Packages to run Gammu.