Gorno-Badakhshan Autonomous Region (GBAO) is an autonomous region in the east of Tajikistan. Located in the Pamir Mountains, it makes up 45% of the land area of the country. Pamir highway goes through the area and if you are travelling to Tajikistan you are very likely to visit the area.
To visit GBAO you will need a special permit referred to as GBAO permit.
This permit can be applied fro at the same time as the visa and is issued along with the visa if requested (although at an additional cost). It will be valid for the same dates as the visa.
It looks like a large blue rectangular stamp placed next to the visa in your passport. make sure that the stamp is there when you check your visas.
This article was taken from the Visa Machine's advice pages.