After watching the video, I do not think it's a gas explosion. A gas explosion would have made a shockwave, and the orange barrier on the other side of the street would have gotten blown back, or at the very least wobbled around, but it did nothing until debris hit it.

The more gas fills an area before it gets ignited, the bigger the explosion. The way gas fills an area isn't like filling a gas tank in a car. It it was natural gas it rises to the ceiling and hangs around pushing slowly downwards as it tries to fill an area by forcing out the air as it's replaced with gas. Natural gas is lighter than air, and that building has HVAC. The duct work would have help pump it around the building so others could detect it by the smell. But that explosion is contained in one area.
During the gas leak, someone could have smelt the rotten egg smell they put in natural gas to warn them of a leak.

So either the gas didn't have time to fill the room, or it wasn't a gas leak.