I tried using the 8.8.8.8 one for one domain, but sending emails from Gmail to that domain failed for hours, so it's not perfect. One would at least think that Gmail uses 8.8.8.8 for DNS.
Was it failing at GMail before you flushed at 8.8.8.8? Because then you would have to wait while GMail's own cache expired, assuming they even use the public 8.8.8.8 service.
Domains themselves don't use DNS servers in the same way your network connection needs DNS to work right. Did you try using 8.8.8.8 for your domain's nameservers? Because that's a misconfiguration - your domain's nameservers need to be configured to be authoritative for your domain - which Google's HonestDNS is not going to claim to be. (Even if you're using GCP.)
I think he meant he used the Flush Cache functionality of 8.8.8.8 but emails still failed on Gmail until their cache was invalidated (so Gmail probably is not using 8.8.8.8, or the flush cache doesn't actually works as intended).
caches are hierachical. Even your local machine most of the time now with recent OS releases will cache records, then your home router or some other DNS server on your network will often cache things before then referring to your ISPs or Googles DNS server.
Invalidating the cache at one doesn't invalidate the cache downstream of them if they already looked the record up recently. But it does mean that anyone who hadn't looked up the record will get the correct result straight away.
And it's not like they need to hold on to any state to work. If you had access you could purge everything and have them start fresh from the root servers, and it would work fine. (As long as the load spike doesn't make it decide to do something dumb, of course.)
Google's 8.8.8.8: https://developers.google.com/speed/public-dns/cache
Cloudflare's 1.1.1.1: https://1.1.1.1/purge-cache/