Apply GPO to specific users on specific computers

I'm trying to apply a GPO to specific computers but only to specific users who access these computers.

I've enabled loopback processing in this GPO and linked it to an OU that contains the specific computers. If I leave security filtering to Authenticated Users, the GPO is applied to any one logging into those specific computers without issue.

To then apply to specific users, my assumption was to simply replace the Authenticated Users Security Filter with the Security Group containing the specific users I want this GPO to apply to. Unfortunately the GPO does not get applied when trying this.

Is there something else Im missing? Another way to accomplish this?

Thanks!