Hello,
The Grigri can be used in multipitch climb as a belay device as long as you respect the technical instruction given in the technical notice.
However, it also suppose that you’re using a single rope (Diameter within the limit…) and that your placing special attention to the device attached to the anchor as the came rotation may be interfering with the rock. For further information, please read the product experience available on the web site :
http://www.petzl.com/fr/outdoor/grigri-produit-experience#GE-multilongue...
We thank you really much for your interest and are staying at your disposal for any further information
All the best