If I understand the above thread, I can use the Gmail App and read my mail (and archived messages) without using the storage on my 6+. That means I can delete the Gmail mailbox in the Native App and thereby free up storage of old Gmail messages. Right now my phone says I have 25,546 saved emails. I do not know how to find out how much memory those 25K messages are hogging. Can you point me in the right direction?
Easy way for you to find out is
1. Look at your storage and screenshot it for your own reference (to refer back to later)
2. Remove your email address from your iPhone. (Settings>Mail>Accounts>Google/GMail>Delete Account)
3. Then look at your storage again and see if it changed. (Settings>General>About> "Available"
If the amount of available storage goes up in number then you found out how much space it is taking up on your phone.
On this screen I see how much space every App on my iPhone is taking up. The screen with all the apps listed shows how much the app and the documents are taking up. (These numbers won't help you unless you delete the app.) You can then click each app icon and see how much space the documents and data is taking up on your phone.
My numbers are
Messages: 1.7 GB
Inbox 14.5 MB
Mail and Attachments 5.5 MB
This makes me think unlike the others have posted that if you are using a 3 party email service there will still be data stored on the device.