No, it is not necessary to install GemBox.Spreadsheet in order to use GemBox.Spreadsheet.dll.
You can just develop your application by adding a reference to the GemBox.Spreadsheet.dll.
You can download GemBox.Spreadsheet.dll directly from the BugFixes page:
Or you can download it via NuGet:
However we do offer an MSI installation for GemBox.Spreadsheet which provides a few benefits, like installation of GemBox.Spreadsheet assembly into the Global Assembly Cache (GAC), local help file (CHM format) with API references and other resource files.
The installed assembly can be easily shared (referenced) by multiple applications and also the installed assembly can have multiple versions which are listed side-by-side in GAC.
When you have a .NET assembly enlisted in a GAC, you can add reference to it inside a Visual Studio by browsing through a ".NET" tab in an "Add reference" dialog. In more recent versions of Visual Studio (2012 and later) the GAC assembly location in the "Add reference" dialog (called "Reference Manager") is under the "Assemblies" -> "Extensions".