I have not found answers for "How to setup Parental gate for game developed in UNITY3d" in Unity community, its possible using alert view in Xcode for iOS but how to achieve the same in unity itself?
Visit https://four-iq.appspot.com/docs/sdk-unity/#/parent-authentication
This site provide a Unity SDk, integrate with the scene where in the in app purchases are to be provided parental gate.
Follow the instructions provided and thus the parental gate is setup for Unity game developed for iOS devices.