The dark circles around gaara eyes are probably because of kohl.
Kohl is basically just eyeliner. The peoples from sunny/desert regions - hence "sandy" - from around the world used to use this for eye protection.
A quik wikipedia search gave me following info:
Kohl (Arabic: كُحْل) is an ancient eye cosmetic, traditionally made by grinding stibnite (Sb2S3) for similar purposes to charcoal used in mascara. It is widely used in the Middle East, North Africa, the Mediterranean, Eastern Europe, South Asia, the Horn of Africa, and parts of West Africa as eyeliner to contour and/or darken the eyelids and as mascara for the eyelashes. It is worn mostly by women, but also by some men and children.
Also interesting is this:
In addition, mothers would apply kohl to their infants' eyes soon after birth. Some did this to "strengthen the child's eyes", and others believed it could prevent the child from being cursed by the evil eye.
I hope this answers your question. I think its just a cosmetic/cultural thing they gave to gaara. To make him look like the desert tribe peoples.