[Hurdy]

When you go through the sign up flow you can select whether you want to Gmailify your address into an existing or a new Google account. We make it very explicit that all your mails are being copied into Google servers.

By the way, while Gmail always has offered ways to just "import" your messages from a third party address, with Gmailify you get a full two way sync, so you can e.g. keep using the Yahoo UI and your changes will show up in Gmail.

[OxO4]

I do not seem to have the option to gmailify my address. Is the feature restricted to specific providers? If yes, are there any plans to support Office365 addresses?

[Hurdy]

It's currently limited to consumer accounts on Outlook.com and Yahoo.

[ikeboy]

Can I gmailify my other gmail accounts into my main one and get two way sync?