How can I rappel with a Gri-Gri?
Use the biner-block method, also called a Reepschnur rappel, to descend on a single strand. This method actually works with any belay device you choose: sticht plates, tubular devices, assisted braking devices, or Figure-8 type devices. It's most commonly employed with Gri-Gris because such devices don't function with two strands.
Feed the rope through the anchor to its midpoint.
[Recommended] Tie knots in both ends.
Drop the two ends below or holster coils on your harness.
Tie an overhand on a bight of the rope near the midpoint, on the strand of your preference.
Clip a locking carabiner from the overhand loop to the bight of rope on the other side of the anchor.
The section of rope with the knot is your pull strand. The other is your rap strand. Pull the rap strand to make sure it blocks securely against the anchor and doesn't move. Test the pull strand to make sure it will run freely.
Attach your belay device to the rap strand and to your harness, following its directions for use.
[Recommended] Extend your rappel and back it up with an autoblock or kleimheist knot.
Don't be HAD. Double check your harness, anchor, and device.
When all persons have rappelled, yard on the pull strand to retrieve the entire rope. If more rappels are to follow, remember to pre-thread the next anchor or otherwise secure the rope before pulling.