Gate quest is a quest that unlocks access to new instance where another quest content is located.

Keys ideas:
1.The level behind the gate/portal unlocked is instanced random(procedurally generated usually), not connected with open-world normally and provides a unique chance to reach it.
2.The level entrance appears normally as some static object in open-world, but when you complete the gate quest, it becomes active(player may be informed of gate location).
3.Once you complete the gated instance it disappears forever(i.e. quest end).
4.Portal use in/out of the gated instance is usually limited if not disabled.
5.The objective of first part is to unlock access to something*(player has only vague idea). (unlike item-based consumable portals to X which are strictly defined and repeatable)