Hi Ido,
As noted already, there is no way to workaround this in the Gateway side. I can imagine a convoluted workaround though (this is untested though) where you route to something else (like a load balancer VIP) that you would control and terminate SSL on it so that the Gateway then won't bother with the backend SSL certificate, and that load balancer could be configured to ignore the validation of the backend certificate. This workflow, I believe, would allow you more control to have a long-running expiry date for a certificate so that you do not have to worry as much about the Gateway running into certificate validation issues. This should alleviate the frequency of these issues, if not altogether eliminate it as you'd have more control.
With that said, between the backend being the root cause (and needing to "up their game" for properly replacing certificates well in advance of their expiry) plus the security issues that can arise from a feature that tells the Gateway to ignore all certificate validation, I personally think it's not worth putting in that feature to simply ignore all certificates. But of course, that's just my own personal opinion, not necessarily the opinion of CA Technologies. So please do feel free to create the feature request as an Idea in this community if you feel that feature should be available to users of the API Gateway.
My personal and preferred method forward would be to just greatly improve the process that seems to be allowing the certificates to expire with frequency in such a critical backend system.