Review the system requirements for details on what is required to run PLUSManagedGui. Since PLUSManagedGui depends on PLUSManaged, be sure to read about deploying applications protected with PLUSManaged.
The PLUSManagedGui should be installed in the same directory as your application (or .exe file). This greatly simplifies deployment and also prevents your application from using other versions the library that may not be compatible with your application (which could be installed by other applications that use PLUSManagedGui).
Although installing the PLUSManaged library to the application directory is strongly recommended, it is possible to install the assembly to the Global Assembly Cache (GAC) if required by your application.
If your application requires you to install dependencies in the GAC, make sure your application's reference to any Protection PLUS 5 SDK libraries (such as PLUSManaged or PLUSManagedGui) has the Specified Version property set to true.
The Protection PLUS 5 SDK .NET libraries (such as PLUSManaged and PLUSManagedGui) are obfuscated, but this alone cannot prevent a hacker from modifying your application's source code. When your .NET application is compiled, it is compiled to what is called Microsoft Intermediate Language (MSIL - pronounced like "missile") code. There are tools that can be used to translate and reverse engineer MSIL code, so it is imperative that you select and use an obfuscation tool to further protect your application and your investments in it. Microsoft offers a good summary of this subject.
It is possible to obfuscate your application in a way which embeds the Protection PLUS 5 SDK libraries with your application. This is recommended where possible, as it works with PLUSManaged and PLUSManagedGui, provides some extra security with most obfuscation tools, and can help simplify your deployment (as won't be necessary to distribute the library once it is embedded in your application).