Privacy/Security concers for setting up business email using Gmail app on Nexus devices
The gmail app is just a client and nothing is stored on Google's servers. The use of IMAP simply means that the email is stored on your company's servers and your phone, as compared to the older POP where the email is pushed to your phone and deleted from the server.

The emails might show up "mixed" if you show all inboxes but you may be rest assured that they do not reside on Google's servers.