It looks like they stopped advertising it a few months ago (and told their affiliate partners to say it doesn't exist), but for a long time the marketing for Gateway was offering the same language about expanded last-seat award availability.

Just did a search and I have XN access with Gateway.

Note that back when the credit card language was specific, no cards had XN as an officially published benefit, only JN and YN. But they've replaced the language on all cards now with a generic message and it appears that YN is available to everyone now, not just cardholders: "Award Availability, Pricing, and Fees: As long as tickets are still available for sale for the applicable cabin on any United-operated flight, MileagePlus members can use miles to book award travel on such flight. Award pricing is subject to change without notice."