Zwift currently has a closed API, which means that it can only sync with major platforms like Strava. Great Cycle Challenge cannot currently connect directly to Zwift.

Any rides completed on Zwift, however, can absolutely count towards your Great Cycle Challenge. You can either add them as a manual ride or if your Strava and Zwift accounts are connected, you can connect your Strava account with Great Cycle Challenge to sync them automatically.