Help centerのFAQにはIMAP Accessの項目があって、以下のように書いてあります。
--------------------------------------------------------------------------- How do I enable IMAP?
You can retrieve your Gmail messages with a client or device that supports IMAP, like Microsoft Outlook or Apple Mail.
To enable IMAP in your Gmail account:
1. Log in to your Gmail account.
2. Click Settings at the top of any Gmail page.
3. Click Forwarding and POP/IMAP.
4. Select Enable IMAP.
5. Configure your IMAP client* and click Save Changes.
ヘルプにのってますね (スコア:4, 参考になる)
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How do I enable IMAP?
You can retrieve your Gmail messages with a client or device that supports IMAP, like Microsoft Outlook or Apple Mail.
To enable IMAP in your Gmail account:
1. Log in to your Gmail account.
2. Click Settings at the top of any Gmail page.
3. Click Forwarding and POP/IMAP.
4. Select Enable IMAP.
5. Configure your IMAP client* and click Save Changes.
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# でも、私の所もまだ"Forwarding and POP"のままですけど