No downtime is "unicorn feathers" territory. Extremely low downtime requires redundant systems and a dedicated support team actively trying to head off potential problems. And Gmail still has downtime.

But you're asking the wrong questions. Email itself has retries built in, it doesn't need perfect uptime. What you should be asking is how badly Gmail deciding you might be a spammer and not caring about fixing that for one person is going to torpedo your deliverability rate.