$GatewayServer = Get-SCOMGatewayManagementServer -Name "GW01.dmz.contoso.com"
$FailoverServer = Get-SCOMManagementServer -Name "MS02.Contoso.com","MS03.Contoso.com"
Set-SCOMParentManagementServer -GatewayServer $GatewayServer -FailoverServer $FailoverServer