The main difference among Email and Gmail is, what type of accounts they are supported.

GMAIL::

The Gmail Application is only used & limited for accounts with Google:username@gamil.com. It send’s back
/uses its own proprietary protocol to the Gmail server.
You can read, respond to all your conversations in offline or online.You can also manage multiple accounts (supports only username@gmail.com) save attachments. Apart from that we can even label the notifications.

EMAIL::

This is a very basic APP. The most & main important feature of this app is-it can manage all type of multiple accounts (i.e, username@your_domain.com) but dosen’t support search capabilities like Gmail.
Email APP opens POP3/IMAP protocols by supporting  Activesync.

So if you using only single Google account then it’s better to use Gmail App but if you use multiple accounts or non Gmail accounts use Email. Coming to me I personally use Gmail App.

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