You may have noticed that while creating a new Group Managed Service Account in the Active Roles Web Interface, the search operation on the Membership Policy tab will not return Computer objects. It only returns User and Group objects.
Create and manage Group Managed Service Accounts using the Active Roles Console. Or add the Computer objects via the AR WI post gMSA creatioon.
This will be fixed on a future release of the product. Please refer to this article for updates or contact support referencing the defect ID 91717.