This is what I'm wondering as well. IMAP isn't an option, just POP3.
If I want to use my Gmail account as an 'email client', my only option to connect to an external email address is via POP3, at least in Canada.
Is this the case elsewhere? I don't see IMAP as an option with the 'Check email from another account' option in the settings. Their 'Learn More' button says:
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Add another email account on your computer
In a web browser, at mail.google.com, you can add:
Another Gmail account.
A non-Gmail account like Yahoo or iCloud Mail.
You can add up to five email addresses to your Gmail account.
In the Gmail app, you can also add another email account.
However, you can't add accounts like Exchange and Post Office Protocol (POP). Learn how to add an account in the Gmail app.