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That was one of the problems with 6to4. If there was no gateway or it was overloaded or there was a gateway but you couldn't reach it because of a weird firewall, all your IPv6 packets would be silently dropped and you'd have no idea why. And this was before happy eyeballs so your computer might default to broken IPv6.

I think the intention was to use normal internet (anycast) routing to send it to the closest translator - which would be your ISP, or the nearest ISP that supports IPv6, or a tier 1 network which is happy to have extra traffic traversing its network unnecessarily since they get paid for all of it. (The same reason HE runs the free tunnel broker)

You can configure it statically but there used to be the anycast address 192.88.99.1 and the idea was that you'll get routed to the nearest one by magic of BGP. It was retired once native IPv6 deployment took off.

Apparently the practical problems were related to people haplessly firewalling it out (ref. https://labs.ripe.net/author/emileaben/6to4-why-is-it-so-bad...)