Well you don't add anything to Styles as that is all CSS.
However if you mean Gantry 5 Outline... Say Home or Default or a Custom Outline that you create, then you would go into the Page Settings tab and then in the Head Properties section you will see Meta Tags with a + button. You click on that Plus button and add your Meta Tags there. For example you could add Keywords to the left and then to the right add your keywords...
Then when done make sure you click on Save Page Settings button...
I think that will give you what you want. Good luck.
Best,
Grant