"Three more people died on Wednesday, including an early morning death and two later during the day, taking the overnight death toll to thirteen," a district official said.
Two more are reported to be critical at a government hospital, the police said.
The administration on Wednesday morning ordered a probe after families claimed it to be a hooch tragedy.
The opposition Bharatiya Janata Party (BJP) has targeted the Chief Minister for the tragedy.
Senior BJP leader Nand Kishore Yadav said: "If there is total ban on liquor in Bihar, how come the poor are dying after consuming it? The Nitish Kumar-led government has failed to enforce liquor ban."
All victims of the supposed hooch tragedy were residents of Nonia Tola locality. Families had reported that the victims had complained of uneasiness and breathlessness, followed by stomach pain and vomiting, after they consumed "desi" alcohol on Tuesday evening.
Their condition deteriorated on Tuesday night and later they died in the hospital, the police said.
Rahul Kumar, Gopalganj District Magistrate, constituted a three-member team and ordered a probe into the incident.
"It is too early to say anything about the exact cause of the deaths," Kumar had said.
Gopalganj Superintendent of Police Ravi Ranjan had, however, refuted the rumours of spurious liquor consumption.
"It is a matter of coincidence that they died one after another in a span of few hours," Ranjan said.
The incident has exposed that despite total prohibition on liquor consumption in Bihar since April 5 this year, alcohol is available in the black market.
Additional Director General of Police Sunil Kumar said at least 4,707 people were arrested during the last four months and 3,719 FIRs were lodged in connection with flouting of the liquor ban.
