Baki Tezcan writes in an LA Times opinion piece:
In the 1920s and ’30s, Kurdish citizens of Turkey were brutally massacred in military responses to rebellions. Those uprisings were also conveniently used by the ruling regime to suppress political opposition and further consolidate the single-party system. In the most savage case, Turkish security forces used aerial bombings and mass executions, and in some instances poisonous gas, against Kurdish civilians in the 1937-38 rebellion in the Dersim region. Many of the survivors were moved to other parts of Turkey and Kurdish girls were given to urban Turkish families for adoption.
I see however that Wikipedia is much more reserved about this, stating only that
Nuri Dersimi claimed that the Turkish air force bombed the district with poisonous gas in 1938.
(Dersimi is a Kurdish writer.) So what evidence is there for the claim that Turkey used gas against the Kurds in the late 1930s?