Yes, backing up Gmail messages into MBOX is possible. You can do it both manually and automatically, depending on your necessity.

Manual Method:
The easiest manual option is to use Google Takeout. Here’s how you can do it:

1. Go to Google Takeout (just search for it on Google).

2. Sign in to your Gmail account.

3. From the list of services, select only “Mail” if you only want to back up your emails.

4. Choose the export format — by default, it provides emails in MBOX format.

5. Complete the process, and Google will send you a download link once the archive is ready.

This works great if you want a one-time backup. The MBOX file you get can be opened with email clients like Thunderbird, Apple Mail, etc.

Automated Approach:

During continuous backup or managing huge mailboxes in a smoother way, there are available programs on the internet that can easily transfer Gmail to MBOX without wasting much time. You can also filter emails, back up at given specific times, or just export the folders of your choice using some of these tools, thereby saving time in case you do not wish to download everything.