GAC + sharepoint powershell

[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")
$publish = New-Object System.EnterpriseServices.Internal.Publish
//Add your dll folder
$publish.GacInstall("c:\temp\MyDLL.dll")
iisreset