You should probably find Gaupol via your distro's package management, if not, get the tarball and install from source. The latest release is 0.28.2. A list of changes can be found in the NEWS.md file. You can try Gaupol just by running it from the source directory. For instructions on running and installing, please read the README.md and INSTALL.md files included in the source package.
The latest available Windows-installer is 0.19.1. All necessary dependencies are included. A list of changes can be found in the NEWS.md file.
Experimental installers of some later versions are available as well, but they might be unstable, slow or ugly, mostly due to the experimental nature of GTK+3 Windows packaging in general. Please note that 0.19.1 does not contain features added in later versions, e.g. the integrated video player, and also that the experimental installers lack some features, e.g. the integrated video player, due to packaging problems.