Minecraft wiki entry for End_gateway, associated text: Their generation is determined as follows: The game first draws a line with a length of 1024 blocks from the center (0,0) through the activated gateway on the central island. If the conditions above are met in the chunk at the end of that line, the game decreases the length of the line until either a chunk not meeting the conditions has been found or until the line has a length of 768. If the conditions above are not met, the game increases the length of the vector until it first finds a chunk meeting the conditions. The gateway is then generated in the last found chunk matching the criteria. Knowing this, the position of the exit gateway can be manipulated.