Ah, I think you're a little confused over the terminology! SMTP is the protocol used to SEND mail. IMAP (and POP) are the protocols used to READ your mail once it's been received by your email server.
Gmail has a web interface to both read and send mail with a normal browser. But you can also hook up an email client application (running on your phone or computer) to both send and read mail from your gmail account. The protocols used by your mail app are SMTP to talk to a server to send mail, and IMAP to talk to a server to read your mail.
For example I use the "Mail" app on my Macintosh (and iPhone) to connect to several email servers. From my own server, to iCloud and Gmail. The app uses the SMTP service on those servers to queue mail to send, and IMAP to talk to the mail services on those servers to suck up my incoming email and copy them to my local machine.
IMAP is the modern replacement for POP (post office protocol). While POP worked ok for many years, it has been rightfully deprecated by all modern email servers. It's just much easier to manage your email with IMAP.
Both SMTP and IMAP were designed to run on the early Internet (before encryption) but have modern flavors where the traffic between you and the email server are encrypted from casual eavesdropping (e.g. by your ISP watching the packets flow across their routers).
But one problem with most email is that the messages themselves are stored on the server in clear-text (not encrypted). So if you're doing really secret work, you need to accept that the owners of the email servers (and/or hackers that break into them) have full access to whatever email is stored on their computer.
And that's where secure email comes in. Using PGP (aka GPG) keeps the email encrypted until you decrypt it on your own device (iPhone or computer), so you don't have to trust the owners of your email service.
Hope this helps!
= new reply since forum marked as read