Minecraft wiki entry for End_gateway, associated text: If an end gateway is broken (e.g. in creative), players can still enter the gateway on the other end. This teleports the player to the broken portal, which does not regenerate.
Once an end gateway generates on the main island, the corresponding gateway in the outer islands can generate, even if the player has not yet entered the portal on the main island, or the section of the outer islands has already generated without the gateway.
Using the exitportal tag and the /setblock command it is possible to create a gateway portal in the Overworld and program it to go to a location.