Abdul Manas, the chief of the district said the attacker was on a motorbike.
Manas said: "Twenty-seven bodies have been taken to the hospital and among them 13 are police."
Residents in Gereshk have experienced some of the worst violence from Taliban fighters.
The number of attacks have grown steadily, particularly in the past two years.
More than 100 suicide bombings are said to have occurred this year.