To install DLL into Global Assembly Cache (GAC) you need Visual Studio or SDK to run gacutil.exe. It’s not convenient when you need to deploy ASP.NET application in IIS. This time you can do this with Powershell as described in TechNet article How to Install a DLL to the GAC on Windows Server 2012 Using Only PowerShell by Muhammad Khalid Latif.
Set-location "c:\deployment folder"
[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=220.127.116.11, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")
$publish = New-Object System.EnterpriseServices.Internal.Publish