We are designing a cross platform application for learning foreign languages. Some lessons in the application will be free and others will be paid. So I'd like to know is it possible to use 3rd paty payment gateway to sell lessons or I have to use in-app purchase via AppStore?
Is it allowed to sell content outside the AppStore
The guidelines are here:
https://developer.apple.com/app-store/review/guidelines/
Note that if you are unlocking code in your app then you must do that through IAP.
You might be able to operate as a 'reader' app under 3.1.3