Thank you for your interest in our internship / training program. If you're reading this message, it's because you are wondering why we ask for a Gmail.com account.
The majority of our applications use the Google API. As such, a proper "gmail.com" account helps avoid compatibility issues if/when you are hired.
Entering your gmail.com email address in the self-assessment form avoids us having to ask for it later.
NOTE: Feel free to enter whatever email address you would like, but if you are hired you will be asked to provide a "gmail.com" address later.