Move Large Apple Mail .mbox Folders to Gmail

I have e-mail from accounts that no longer existed archived in Apple Mail, in their .mbox format. Does anyone have recommendations on how to move the messages to Google Mail, so they'll be more accessible?

I have read that the format that Apple Mail stores and exports its messages is not actually standard MBOX format. It is a proprietary format in which the folders end in ".mbox", and contain directories within them named "Attachments" and "Messages", along with a file named "Info.plist". The Attachments folder contains a bunch of other folders with numbered names. The Messages folder contains files with numbered names and the suffix ".emlx". The numbers of the Messages file names and the Attachments folder names appear to match. If you suggest a utility that handlex .emlx files, will it preserve the attachments in the other directory?

Unsuccessful Solutions

I've tried dragging and dropping within the Apple Mail application, but it doesn't transfer all of the messages. There are tens of thousands of e-mails in these .mbox folders, which seems to just be too much for Apple Mail.

Google Email Uploader (http://ift.tt/1bzgKl1) doesn't upload to Gmail accounts (oddly enough) and also doesn't read the mbox folders correctly.

IMAP Upload (http://ift.tt/1cYa8Rz) doesn't work either.