What it does
I don't have reliable information about a Magic: The Gathering keyword or ability called "Eternity Gate." This doesn't appear to be an official MTG keyword, ability, or mechanic that I can confirm exists.
It's possible you're thinking of something else—perhaps a specific card name, a mechanic from a recent set I'm not certain about, or a term from a custom/unofficial format. Some possibilities to consider: "Gates" are a land subtype (relevant to cards like Maze's End and Gond Gate), and "Eternalize" is a real keyword from Amonkhet that creates 4/4 black token copies.
If you can share the exact card name or where you encountered "Eternity Gate," I'd be glad to help clarify the actual rules and how it plays in Commander. I'd rather admit uncertainty than invent rules text that could mislead your gameplay.
Iconic cards with Eternity Gate
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