Minecraft wiki entry for End_Gateway_(block), associated text: ExitPortal tag holds a set of X/Y/Z coordinates to determine the general location the player can be "safely" teleported to when entering.
Setting ExactTeleport to 1 teleports entities to the specified coordinates exactly instead of at a safer location around those coordinates.
Age determines how long the gateway has existed. This is used to determine when the beam is rendered.
Example: /setblock ~ ~-1 ~1 minecraft:end_gateway{Age:180,ExactTeleport:1,ExitPortal:{X:1,Y:2,Z:3}} replace