Configuring two NS entries is pretty standard, so surely most resolvers try one of the two, and if it's down try the other one? What else would be the point of having multiple nameservers? Then you just have to get two nameserver providers and make sure their settings stay synced, and point your domain to one nameserver from each.

Of course that requires the server to properly fail, i.e. stop responding to requests. That doesn't seem to be the case here

You set both services in your ns records. So every day they share the load for dns resolution. If one day one of them is down the client can/will use a different nameserver from your configuration.

Have them all hot and live rather than any sort of failover system. Keep everything in sync with OctoDNS or similar

https://github.com/octodns/octodns

DNS is fastest first* rather than main/failover. If AWS DNS was down your GCP DNS would have replied (if all is well) sooner than {timeout} so your visitor would still have a response

* Sort of. I think if the client doesn't get a reply from the server it picked randomly in 1s they move on to the next server, repeat until all fail

Ibthink if route53 was down. Your dns provider whouldn't able to go there. So it will go to the root who will give gcp one too. So your dns provider might try that.

(I don't know if this is how it works, but I thibk that's how it supposed to work)