How to revert the default mail client back to Mail from GMail

Firstly, if you're using Gmails two-factor authentication, you'll need to setup a one-time password for Mail.app to use. From the look of it, you are not using this feature but you probably should and so should everybody else so I'll include the steps for completeness. You can create a new one-time password by going to your Google Account settings (not Gmail settings). You'll see a section like the one in the image below.

Click on the 'Edit' beside 'Authorising applications & sites' and enter your google account password when prompted. You'll be brought to a page where near the bottom you can name the app you're creating a one-time password for and then click create password. See image below. Your password will be generated, spaces in it don't matter so you can just select it and copy to the clipboard. Then open the Mail.app and if you don't get the dialog for adding an account, you can bring it up by clicking Mail > Preferences and you can click the '+' symbol at the bottom-left to add a new account. Put your gmail address, including 'gmail.com' in the 'Email Address' field and you newly generated one-time password in the 'Password' field and hit Enter. Mail will connect to the server and validate then setup your account.
Click 'Create' when prompted and you're ready to go.

Now, to manage the mail that your Mac will download and store locally you go to the Advanced tab for the account. There is a dropdown (pictured below) which allows you to select what Mail will download.

If you don't want to download all mail and attachments, change the default to one of the other options shown below.

• If you select 'All messages and their attachments' this will download every email you've ever sent/received which still exists in your online gmail account. This is the default and probably the reason you exceeded your limit. This should be a one-off problem though if you downloaded them all and this is what you want.
• If you select 'All messages but omit attachments', it will still download every mail you've ever sent as above, just without the additional bulk of attachments.
• If you select 'Only messages I've read' then Mail won't download unread messages, leaving you to read your unread mail on gmail in your web browser.
• If you select 'Don't keep copies of any messages' then you will only ever see mail in your Mail app when you are online. Essentially this will function as if you're using your web-based email account on gmail.com but through the Mail app. This means you don't use lots of storage on your Mac, but it also means you need to have an internet connection to use your account.

I hope that clarifies your choices and helps you choose the best option which suits your needs. Also, if you're going to use IMAP, which you probably should, you should stop using POP.

In GMail settings, you can also limit the number of mails which can be held in an IMAP folder to various limits between 1000 and 10000

In Mail.app 9.3 (and possibly earler versions—I've never tried) this can now be done fairly easily—more or less, anyway:

1. In the mailbox list, beside "Smart Mailboxes", click "Show" then click "+" to create a new smart mailbox.

2. For "Important", select messages that match all of the criteria "Message is in mailbox Inbox" and "Message is in mailbox Important".

3. For "Starred", create a new smart mailbox that matches all of "Message is in mailbox Inbox", "Message is not in mailbox Important" and "Message is flagged":

4. …or, if you're like me and instead have Gmail show "All Starred", you can use the "Flagged" mailbox. This isn't quite the same as the (misnamed) "All Starred" section of Gmail's Priority Inbox view, which excludes starred items already shown in any of the sections further up the page (notably: threads which are both important and starred), so if you want to be really pedantic, create a new smart mailbox that matches "Message is Flagged" and "Message is not in mailbox Important" (and make sure that's the smart Important mailbox; select the regular Important mailbox and you'll unintentionally exclude important starred messages not in the inbox!)

5. For "Everything Else", create a new smart mailbox that matches all of "Message is in mailbox Inbox", "Message is not in mailbox Important" and "Message is not in mailbox Starred":

6. Finally, drag these new smart mailboxes to the favourites bar at the top of the window:

Caveats:

• Mail.app will only show one of these mailboxes at a time, whereas Gmail's smart inbox will show up to four different sections all at the same time.
• Because Gmail keeps track of labels and important status per-message (even though they are displayed as if they apply to the whole thread), you will occasionally see threads appearing in more than one of these smart mailboxes at the same time.
• No guarantees about what happens to important status when you start dragging messages to other mailboxes!