There are a couple of possible confusions in this question. Copying existing email to Gmail can be a very useful thing. I’ve done it myself. But I want to make sure you know exactly what it means.
Programs versus accounts
Outlook is an email program. (To be clear, we’re talking about the Outlook that comes with Microsoft Office. That is completely unrelated to the Outlook.com website web service.) Outlook, the program, is not your email account, and it’s not your email address. You can use Outlook to download email from any email service to your PC. You can use Outlook to download your ISP provided email; you know, where your address is something like firstname.lastname@example.org. You can use Outlook to download your Yahoo mail, your Gmail or mail sent to just about any email address you might have.
That’s what configuring an account in Outlook is all about. You’re telling it what email service you’re using and how to go about fetching email from that service.
Gmail, on the other hand, is first and foremost, an email service. That means, for example, you’d have a gmail.com email address.
Perhaps you already have Outlook configured to access email using your Gmail account. Fantastic.
Using Gmail for other accounts
Now if you have another email address, you can actually use Gmail to act, kind of, like an email program. In Gmail settings, under accounts, there’s an option to check mail from other accounts using POP3. You would configure this to fetch the email from your existing email account, much like an email program on your desktop might. That way, you would read newly received email in Gmail’s interface instead of Outlook.
The problem, of course, is what to do with all that old email that you currently have stored in your PC, in your Outlook folders. It’s really just a two-step process.
Moving existing email into Gmail
Step one, configure Outlook to access your Gmail account using IMAP, just as if you were planning to use Outlook to read your Gmail for everything. In reality, it’s a short-term thing because…
Step two is to copy by dragging and dropping email from your previously existing folders into the Gmail folder that Outlook will have created. You can even create or move sub-folders in there if you’d like. This will actually cause all of the email to be uploaded to Gmail. (It may take some time depending on your internet speed and the amount of mail.) You’ll eventually find it available to you in the Gmail web interface – or in fact, on any device that you connect up to your Gmail account using IMAP.
Once that’s done, you can then access your email on the web using the Gmail interface, or you can leave Outlook running with it configured to access your Gmail account via IMAP; two different ways to access the same single email account from multiple places.