To get to GBAO first of all you need permit which is small paper or stamp on your passport or in some case on your e-visa papers it depends where you will adress for this paper.

Why you need extra permission apart visa to get to GBAO? GBAO is abbreviations of Gorno Badakhshan of Autonomous Region and since Soviet time this paper was obligatory for this region due to its borderland with China & Afghanitan. 

Price: If you apply through embassy & online it will cost you arounf 20$ if through OVIR (Police station department) then it will be around 3$.

Registration: Since January 2022 Tajikistan announced visa free regime but in additional to it they now require registration if you stay more than 10 working days. 

How to apply for registration? Ask you hotel/ guesthouse where you stay, or tour agent.