Here's my take on this:
If you were not holding a cell phone, you were charged under the wrong subsection. GPS devices fall squarely under the section of display screens. See below.
Exempt devices
7. The display screens of the following devices may be visible to any driver in a motor vehicle driven on a highway:
1. A device that displays,
i. information on the conditions, use and immediate environment of the vehicle, or
ii. information on road or weather conditions.
2. An ignition interlock device.
3. A car audio control that displays only text or static images.
4. A hand-held device that displays only text or static images and is connected directly into and operates using the audio system controls of the motor vehicle. O. Reg. 366/09, s. 7.
Requirement that display screens be secure
8. The exemptions in sections 2, 3, 4, 5, 6 and 7 apply only if the display screen of the computer, mobile data terminal or other device is placed securely in or mounted to the motor vehicle so that it does not move while the vehicle is in motion. O. Reg. 366/09, s. 8.
A GPS device, while capable of sending and receiving information/data, is covered specifically under the def'n of a display screen - for which there is an exemption if it's attached to the dash or windshield.
There is an exemption for the use of cell phones in that you are permitted to press 'a button' to send or receive a call, if the phone is similarly attached to the dash.
If you were pressing a button on the GPS, that's different than entering a whole address. It's still distracting and may get you in trouble.
Best bet is to tell the truth - you were charged for a cell phone offence, and you were not using your cell phone. Period. The prosecution may try to amend the charge to the correct subsection, and it will get a little tricky to wade through the legal mumbo-jumbo. If you're dead set on maintaining your perfect record, you may have luck with a paralegal service on this one.