Change the UserGroups in the default RD CAP and RD RAP using powershell?

I'm trying to change the user group membership that is allowed to accesses the Gateway server.

I am trying use set-item but I get the following:

PS C:\Windows\system32> set-item RDS:\GatewayServer\CAP\RDG_CAP_AllUsers\UserGroups "testgrp1@DOM1"
set-item : Access to the object at RDS:\GatewayServer\CAP\RDG_CAP_AllUsers\UserGroups is denied for the cmdlet
Set-Item.
At line:1 char:1+ set-item RDS:\GatewayServer\CAP\RDG_CAP_AllUsers\UserGroups "testgrp1@DOM1"+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : PermissionDenied: (RDS:\GatewaySer...sers\UserGroups:String) [Set-Item], AccessViolation
   Exception+ FullyQualifiedErrorId : PermissionDenied,Microsoft.PowerShell.Commands.SetItemCommand