The firing incident occurred at the Gondka polling station, which left three people injured with two succumbing to their injuries while being transported to hospital.
A heavy contingent of police officials and Rangers personnel reached at the site to restore order.
A PTI Senate candidate from Islamabad Fareed Rehman accused Gilani of concealing his conviction in a contempt of court case back in 2012.
Since a period of five years had elapsed since Gilani's conviction, he stands qualified for contesting election to the Senate from federal capital, ECP ruled.