CA Test Data Manager (TDM) requires a compound license for Datamaker and all other components, including GT Diagrammer. If you do not already have a TDM license, you can request one using a Customer Care Case Request form: https://support.ca.com/irj/portal/anonymous/customercare. Provide your Enterprise Site ID, Product Name (CA Test Data Manager), and release number. When the request is processed, you will receive an activation key by email in the form of a text file. The key should be one long line of alphanumeric characters.
GTDiagrammer is not a standalone component and cannot be launched separately from Datamaker. You will only be prompted for a license when you are launching GTDiagrammer either by double clicking the icon or by executing the GTDiagrammer.exe from C:\Program Files (x86)\Grid-Tools\GTDatamaker\GTDiagrammer. The application needs to be opened from within Datamaker. There is a user story to make GTDiagrammer a standalone component in the future. If you launch GTDiagrammer from within Datamaker, you should not be prompted for a license.