That is because that class depends on .NET 3+ features, so it is in a dedicated project that has a .NET 3 reference. That project is invoked by the business layer (which is .NET 2.0) only GSP detects that .NET 3+ is on the server. This allows GSP to run on a server with only .NET 2.0 but can take advantage of .NET 3+ when on a server that has it.
When GSP eventually targets .NET 4, I will likely move those classes into the business project and delete the WPF project.
Roger Martin
Creator and Lead Developer of Gallery Server Pro