PRO subscribers can use the application without an active internet connection at full volume. However, every fifteen days it checks to see if you have an active subscription and you need to connect to the internet for that. Everything except web fonts, libraries and templates work in offline mode.
Even if you are using free Gravit Designer you don’t need to worry about a loss of your internet connection. In this case, you can still download your files as a backup.
Please notice that this is a PRO feature. In the free version of Gravit Designer, an internet connection is required.