GPS will tell you nothing useful about grade. It's the nature of satellite triangulation that elevation, and by inference grade, are extremely inaccurate.

If you must have an expensive gadget, a barometric altimeter is the ticket, but a $20 bubble inclinometer is the most accurate indication of real-time grade you can buy.

But she doesn't need to know what grade it is when her cadence drops below her ideal. Cadence meter will tell her that...