So I looked into this a little further as it just started affecting me today.
This is the error,
What I gather is the required SSL key was left out of the cert chain (duh). A quick google search gives us,
A few of the key points being,
The good news is that if you’re the website owner and facing this issue, there’s a way to resolve it. And the bad news is that, if you’re a website visitor you can’t do much.
The error itself means that the keys you have tried to pin isn’t bound to the pinned certificate. Also, these keys are not interchangeable, and failing to pin the right key to the right certificate can break the website.
Since other browsers are able to access gmail.com however I'm fairly certain there is a key problem on Opera's side.
As I typed this post it started working again, the only thing I did was try to swap my DNS-over-HTTPS provider from Cloudflare to Google, then back again after about 30 minutes. I'm not sure if there is some sort of delay in the browser switching providers, or if the issue was resolved on Gmail's end.
Either way that's all I have to add at the moment, hard to determine more without looking at the code.