The women were attacked by gunmen on a bike as they were travelling to a hospital to deposit their vaccine kits, Efe news reported.
They had been immunising children in the town of Parmoli in the Swab district in the Khyber Pakhtunkhwa province, police said.
According to local police, the polio workers were not escorted by security guards as the region was not marked as a highly sensitive area.
Jihad Khan, Parmoli police station head, told EFE: "After doing their duty at 15:15 hours both were walking to deposit their kits when two unidentified gunmen riding a bike opened fire from their back.
"The gunmen were able to escape the scene."
Gull was initially taken to the Kalu Khan Hospital in Swabi before being transferred to the Lady Reading Hospital in Peshawar where she died from her injuries, the police officer said.
Attacks on polio workers are common in various parts of the conservative Khyber Pakhtunkhwa and Balochistan provinces.
Unknown gunmen shot dead two policemen who were part of an immunization program in the country's volatile northwest in December.
The pair were killed as they were heading to escort a team of medics in the Dir area of Khyber Pakhtunkhwa province, Dir police station in-charge Faisal Khan told EFE at the time.
Pakistan witnessed a jump in the number of polio cases from 12 in 2018 to 140 in 2019.
Zulfiqar Babakhel, of the Polio Eradication Cell, told EFE: "This year six polio cases have been reported in Pakistan so far.
"Three cases were reported in Khyber Pakhtunkhwa, two in Sindh and one in Balochistan."
Pakistan, Afghanistan and Nigeria are the only three countries in the world where the poliovirus continues to affect the lives of the children.
Those under the age of five are the biggest victims of the infectious disease that can cause paralysis and muscular atrophy of the limbs.
( With inputs from IANS )