So I then again tried changing the smtp settings within Prefs to update to the correct settings, but clicking off within prefs meant that the settings weren't saved. So the next issue was that while it saved my Gmail smtp settings, it didn't save the correct settings, so again this meant I couldn't send emails from that account. The only way to change this was to delete the Gmail smtp account within Mail and then replace it, which I did. So I played with the Gmail settings within Mail's prefs and changed the smtp settings but it would then keep going offline. Despite not changing anything, it suddenly wouldn't send emails from my Gmail account.

No changes to my Mac, no updates, no changes to Gmail and no breaches either. A few days ago, my Gmail started playing up within Apple Mail having worked for months.