Officials put the death toll at 21, making it one of the deadliest incidents in Gaza in recent years, not counting the violence caused by the Israeli-Palestinian conflict.
The fire destroyed the apartment on the top floor of the building, home to the Abu Raya family.
Mohammed Abu Raya, a family spokesperson, told The Associated Press that the extended family had gathered for a twin celebration: one of the children’s birthday and one of the adults’ return from a trip to Egypt.
Abu Raya spoke at the Indonesian hospital in northern Gaza, where the bodies had been taken and where sobbing relatives waited for the funeral procession to begin.
Abu Raya disputed claims that stored gasoline fueled the blaze, saying furniture made from combustible materials was more likely to have accelerated the flames.
“The disaster was that no one came out alive to tell us the truth,” he said. “I don’t think it was stored gasoline.”
Abu Raya said the dead were from three generations of Abu Rayas: a married couple, their five sons, two daughters-in-law and eight grandchildren. The identities of the other victims were not immediately known.
Later on Friday, the bodies were transferred from the hospital morgue to a mosque prior to burial.
Gaza is facing a serious energy crisis, largely due to a crippling Israeli-Egyptian border blockade that has been in place since Islamist militant Hamas took control of the area 15 years ago.
People often store cooking gas, diesel and gasoline in homes in preparation for winter. Home fires have previously been caused by candles and gas leaks.