You probably have the 32-bit version of the System.Data.SQLite.dll in your bin directory, but you are on a x64 PC. Since you aren't using SQLite, you can delete these dll's in the bin directory:
GalleryServerPro.Data.SQLite.dll
System.Data.SQLite.DLL
(The other solution is to get the 64-bit version from the download page.)
To keep these from getting copied to your bin directory the next time you compile, remove or unload the SQLite project from the GSP solution. You don't need it.
Roger Martin
Creator and Lead Developer of Gallery Server Pro