They may be using the paid, enterprise G Suite/Google Workspace Gmail: https://workspace.google.com/products/gmail/.

Can they call someone when there is an issue with gmail? Is there a contract that give them some leverage and guaranties? What is the process in the case of a missing business-critical email being lost and needing recovery?

For business critical operation, leave such answers unanswered can be pretty risky.

Yes, Gsuite has a SLA and telephone support etc.

Now would be an excellent time to ask people who use gmail for business critical operation how well that SLA and telephone support work in practice.

I know for example companies that use email for handling customer orders at their stores. I can just imagine the loss if the system was down for hours a few days before Christmas, or worse, actually lost the data.

They have telephone support, but the agents aren't empowered to do very much of anything.

I am not going to try it. I just restored data from a backup email server these emails get duplicated to.

Yeah, to be fair I've never had to test it.

Google doesn't do support.

yeah, this issue affects gsuite domains as well as @gmail.com