The client is based on the .NET framework version 4.8 from Microsoft. If not already present, you have to install the framework first. Download the Microsoft .NET Framework 4.8 or higher, start the setup program and follow the installation instructions (see http://www.microsoft.com/downloads/ for the latest versions).
Download the zipped installer for DiversityGazetteers from http://diversityworkbench.net/Portal/DiversityGazetteers, extract and install it.