It makes a lot of sense. iOS uses your Gmail credentials for more than just mail (it can also sync contacts, calendar items, and notes to your Google account). So it makes sense to not have it within the Mail app.
Besides, it's not "4 layers deep". It's directly under a top level Preferences heading called "Accounts & Passwords" in Preferences, which should be the first place to look for, well, Preferences.
Third, you could find it also by just typing "accounts", "gmail" etc on Spotlight search. Again, just a click away.
If that's the sort of "problem" iOS has, it's pretty solid.
After finding out that you have to add the account via settings it kinda makes sense, but the first thought is: you have an email account, you have an email app, of course you add the account in the app, why would it be anywhere else? The account and the app could not possibly be more directly related to each other. If you already know how accounts are managed in iOS you will think this is fine, but it is absolutely not intuitive, let alone easy to understand for people who do not know about sso and shared accounts.
Anyhow, this is becoming circular. I guess if you've used an iOS device forever you get used to its quirks, but it makes no damn sense if you use other OSes. Gmail, you can manage accounts within the app. Apple's app, you can't. Makes no sense.
HUH?? This is exactly why Accounts are a global setting.
What you are describing it literally EXACTLY HOW IT WORKS on iOS. Set it up once and it will work in Mail and Calendar