Can't verify identity server for gmail

Hi. I open a MacBook Pro 2016 OS 11.7.10, Safari Version 12.1.3 It's my husband's computer who passed away.

I want to be able to use it, mostly for Garage Band. (He left me a small bedroom recording studio).

First question: Can I update this OS for the latest OS 18 … and at the same time, do I update the Safari browser to the latest 18? It has 8 gig of memory.

I started a Gmail address for him, which has been working fine. Until yesterday. I logged on and I get a message that says "Mail can't verify identity of server <stmp.gmail.com> " How do I fix this? Sometimes I want to email files from his computer to mine (MacBookAir 2021 via his gmail address. Please Help :)

There are some of his programs like Logic and ProTools on there, but maybe I can delete them to make room…?

Thanks!