Promotion vs deployment
Promotion and deployment are different concepts. Promotion indicates the quality of an object and specifies what deployment areas it can be deployed to.
Example: At unit test developers may be testing on their local machines and not deploying to deployment areas until system testing begins. Therefore, promoting an item to unit test means that development has been completed and unit testing is proceeding, but the item is not yet of a sufficient quality to be deployed to a deployment area.
If you do not add a deployment area to a GSL stage, promotion to that stage will not result in any deployment, but the stage still exists. You can add a deployment area later to that stage, and then deploy items to that area.