There's no way it's 5%, but Gmail addresses are a pretty poor indicator to use. Hosting your own email is one of the most difficult sysadmin tasks out there because keeping your server off the blocklists is not trivial; it's becoming more and more common for blocklists to just block entire CIDR ranges if a single IP in it gets blocked. I don't fault anyone for not going down that rabbit hole.

Granted, they could be using Proton or some other alternative. But you still have the same inherent problem that someone else hosts/owns your email data.

I think server/app deployments would need to be just a little more happy-clicky and lower friction and less risk before more non techies would venture into that space. All the "best practices" a sysadmin performs would have to be baked into it or at least be checkboxes that enable them at a cost with simple explanations of each functions cost/benefit in video format. Some VPS providers are slowly going in this direction but have quite a ways to go in my opinion. They have lowered the bar for new techies at least.