The Grid Control fascilities are provided for free for properly licensed products. It means that the database used for the repository does not require additional licences as well as the used application server as far as the Grid Control is the only thing on the database.
Well, I am not quite right - there can be installed for example also RMAN repository and other additional software provided for free by Oracle for monitoring and maintaining Your other properly licenced Oracle software installations.
In general however the Oracle licensing is very misty and incomprehensible. Oracle Enterprise Manager Licensing does not clear about OEM itself, focusing on available packs, plugins, etc