Unless you're talking about a different grigri, the one from Petzl, which is designed for belaying climbers is absolutely a fall rated device.
I think you should reread the technical specifications. It's an assisted DESCENDER, not a fall arrest device. Maybe what you were thinking of was this:
Mobile fall-arrest device - Petzl USA | Tactical
Petzl USA. Installed on a lifeline, the mobile fall arresters follow the user automatically as he moves, whether on an inclined or vertical surface. In case of a shock load or sudden acceleration, they lock onto the rope and stop the fall.
www.petzl.com
Don't end up like this guy:
AAC Publications - Fall on Rock, Inexperienced Belayer, Improper Use of a GriGri – No Helmet
On April 20, a male lead climber (23) was put on belay by his partner (male, 19) with a brand-new Grigri. The belayer had never used a Grigri befor...
publications.americanalpineclub.org