Gravitas One is modeled on the "UK fat face" which is a kind of very heavy advertising type created during the industrial revolution in England. The letter forms are characterized by an attention getting and strong contrast between the very heavy vertical shapes and the thin horizontal ones. The contrast of the design means that it will be most useful when set from medium to large sizes.
These fonts are licensed under the Open Font License .
You can use them freely in your products & projects - print or digital, commercial or otherwise. However, you can't sell the fonts on their own.
This isn't legal advice, please consider consulting a lawyer and see the full license for all details.