Gateway's powers allow him to teleport across time, space, and other dimensions. He is also omniscient and can tap into the Dreamtime, not only for himself, but for others as well, making them astral beings and projecting them to wherever/whenever he wants. He's like the 3 eyed Raven in GOT, but even more powerful.
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Remender was a fool to throw him away like he did. I hope some other writer works around that one day. While it's not like Gateway was an often used, beloved character, he's just too interesting to kill, and with his powers, it would be easy to work him back in.