You're not obligated to tell them about your mental illness, and they're not allowed to ask.
HOWEVER,mental illness is covered by the Americans with disabilities act. This means that in order to be protected under the act, you need to tell their HR department about your disability, especially if you need accommodations (medical leave, time off for dr's appointments, etc). Most large employers have policies in place for medical leave (under the family and medical leave act), and all medical information should be kept private by your lovely human resources department. Another benefit of a larger employer is that they often have harassment policies, so if word of your mental illness gets out and you feel like people are discriminating, you can tell HR and everyone will have to go to "sensitivity training"... apparently it helps them avoid lawsuits. Unfortunately, mental illness often renders people unable to take these steps to protect themselves, so I'd urge keep on top of the meds and therapy and have a plan in place with your MHP + support persons should such need arise.
If you think it's going to be a problem, I would encourage you to browse the National Alliance on Mental Illness website (nami.org). They're great.
And I wouldn't worry about the GPA. Nobody cares. Just say you were busy and working and that you learned a lot.