Identity Management, Leaving the University, Migrating your Data
Google Apps Emails and Drive Data
If you have a personal Gmail account, you can use Google Apps @ Illinois, Exporting Data Using Google Takeout to export data from your Google Apps account to the personal account.
If you want to simply download a copy of your files, see Google Apps @ Illinois, Cloud Storage, Downloading your files.
Setting up email forwarding
If eligible, see Email, How to set up email forwarding.