The 63-year-old victim is being treated for burns covering 80 percent of his body after the building, located at 87th Avenue and Dowling Street, was rocked by the explosion just after 8 p.m., Battalion Chief Melinda Drayton said.
A spokeswoman for PG&E said the company's gas inspectors don't believe the explosion was caused by a leak in their equipment.
"There have been no leaks or compromises identified," PG&E spokeswoman Jana Morris said.
Morris said PG&E tested its lines and equipment outside the home; equipment inside the home is the homeowner's responsibility, she said.
The explosion caused the second story of the building to collapse and shattered the windows of a neighboring property, according to Drayton.
Firefighters initially responded to reports of a structure fire and arrived on scene at 8:04 p.m., Drayton said.
She said they were told three people were trapped in the building, a report that turned out to be false. Firefighters instead found one occupant in front of the home who had been badly burned and was being treated by neighbors.
The man was taken to Highland Hospital, where he was in critical condition Monday night, Drayton said. He lived on the ground floor of the building.
Next door, three adults and one child were displaced because there was so much broken glass and debris, Drayton said. Their building does not appear to have sustained any structural damage.
About 25 firefighters responded to the fire, which was controlled at about 8:20 p.m., Drayton said. No other injuries were reported.