This is a former synagogue, which after restoration is now used as a cultural centre. That is: in 2014 during a vist I could freely make.

I tried to find more information in the Wikipedia, which has "Gaziantep Synagogue, also known as the Great Synagogue of Gaziantep, is an abandoned synagogue located in Gaziantep in the eastern region of Turkey. It was closed after the last remaining members of Gaziantep's Jewish population left the city in the 1970s and was in a state of disrepair. Due to teamwork of the Jewish Community in Turkey and the government, the synagogue was restored in 2012. Since then, the military in Turkey has taken it over as a temporary base. The synagogue was a two-story, stone building large enough to accommodate several hundred worshippers. The date of its construction is unknown"

The part about the military using it seems to be wrong. I corrected it, and inserted information that in december 2019 a Hanukkah celebration with 200 people was held here, as I read in the papers.