Well, John. GMail's "crash" was the first issue that has ever effected me, and I have one of the earliest accounts with them, when you got one "by invitation only" through one of our clients back then. And I am using 7 different GMail accounts right now, with different purposes for each one. Given all the other email providers that are also used among all my clients (beside a few that still host their own email server, for various reasons), GMail has been the most reliable, ever...
Email in general is guaranteed to some level, as in cases a server is not reachable, a sending server will retry to deliver an email in increasing intervals, for up to 48 or 72 hours. The GMail (actually also affecting YouTube, Instagram and other Google services) incident was really unique in scope and randomness of the effected services. I "lost" probably about two dozen emails, mainly from a mailing list (coincidentally one that deals with system admin issues), as that server was not (properly) configured to resend (temporarily) undeliverable emails, both out of capacity reasons as well as an ill-fated attempt to prevent spewing spam. That were two dozen messages out of 200-300 I receive on a slow day (up to a thousand on days after an event like this), and all more important emails on those 7 accounts either made it randomly through or were resend after the Google issue was resolved (which took a bit longer to track down not only due to the randomness but also due to red herring error messages by the sending email servers)...
Ralf