On the page
Downloading and Installing WinSCP .NET Assembly:
Use correct gacutil.exe for your version of .NET framework:
• For .NET framework 4.0, use gacutil from Windows SDK 7.1 (or 7.0): C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1\bin\gacutil.exe;
• For .NET framework 3.5, use gacutil from Windows SDK 6.0: C:\Program Files (x86)\Microsoft SDKs\Windows\v6.0A\Bin\gacutil.exe
I'd like to suggest that this page also mention which gacutil should be used for .NET frameworks after 4.0.