Gabion cage retaining walls: drainage and deadman needs?
There are two types of retaining walls: 1) cantilevered, and 2) gravity. 

1) Cantilevered retaining walls rely on a wall that has a compression side and a tension side. The compression side relays on the compressive strength of the concrete, masonry, etc. The tension side relays on reinforcing steel. 

2) Gravity retaining walls rely on the weight of materials pressing down more than side forces pressing from the side. There are a few issues with this system, including sliding, rotation, long term deterioration of the cage, etc. all of which are variables due to the type of soil. The reason there is very little information about these retaining walls is because any change in the soil can have significant affects on the wall. 

So, these walls are generally oversized. 

With regard to drainage from behind the gabion retaining wall, water is a great force that can destroy the wall. Therefore, more drainage (including filter fabric) the better. 

With regard to the deadman, it’s not necessary for 2’ high retaining walls. In fact, it’s not even considered a retaining wall until 4’ high. At 2’ high, there isn’t enough pressure (force) on the wall for concern.