Google has 2 new open source tools which will help you to import their existing email into Gmail using the Gmail API Mail-importer and Import-mailbox-to-gmail.
If you have your old mail archive from your old email account and want to move those old messages to Gmail so that you can have access to your account by making use of Gmail App on your mobile phone then Mail-importer is for you! This helps users to import mailbox files from their computer to Gmail It has command-line support.
For those who want to import all their users old mailbox files into Google Apps for work then “import-mailbox-to-gmail” is probably for you. It is a admin managed tool so other user will not be able to import mails individually. It is a python script that will do bulk imports of multiple email accounts in a domain.
For developers, import-mailbox-to-gmail can be used as an example of how to perform gmail console for Gmail API. Mail-importer shows how to use batching and entries to efficiently query the Gmail API. In the early development stage there is no GUI for these tools but they are all command-line-based.
So, make use of such tools for free to import all other emails into Gmail to get access to everything. Also these two tools are available on GitHub for free.