Crews from Kent Fire and Rescue Service worked through the night to tackle the blaze at Estuary View Business Park.
Stephanie Harris, who was in the area, said there was a "sudden" and "huge explosion" at about 22:00 BST on Friday.
An engineer who was repairing the gas main at the time is in hospital after suffering burns in the explosion.
Another man was treated at the scene of the blaze, which has now been extinguished.
Southern Gas Networks said the exact cause of the explosion has not yet been established.
It said 600 homes in the Old Thanet Way area were without gas and it was working to restore the supply.
A spokesman said: "It is too early to know what caused the fire last night but an investigation is under way.
"We would like to reassure the local community that we are continuing to closely monitor the situation to ensure their safety."
Kent Fire and Rescue said the gas main was isolated shortly before 03:00 on Saturday.
A spokesman said: "It is not known what caused the explosion at this stage."

At the beginning of the season, Claudio Ranieri's side were as much as 5,000-1 to finish as champions, while the odds were 1000-1 to be top at Christmas.
The Foxes lead the Premier League by five points from Tottenham and have lost just three times all season.
Leicester can guarantee themselves a shock title triumph with three wins from their remaining four games.
Betfred are the only bookmakers to have already paid out on Leicester as champions, after they beat Southampton 1-0 on 3 April.
"I've been wrong before but this time I can't see Spurs or Arsenal catching Ranieri's men," said Betfred boss Fred Done.
At the start of the season, 47 people placed a bet with Ladbrokes for Leicester to win the title at 5000-1 - of which 23 have cashed out, while 24 are still awaiting their fate.
"Leicester's win would be the biggest upset in the history of betting," Alex Donohue of Ladbrokes told BBC Sport. "It would be the worst result for us financially and our biggest payout by a distance.
"If Chelsea, Manchester City or Manchester United would have won, the total payouts would have been in the few hundred thousands.
"Leicester have been upsetting the odds all season but it is a brilliant Hollywood story that we are a part of. Hats off to anyone who backed them at the start of the season."
Last month, a Leicester fan cashed out for £72,000 from a £50 bet on his team to win the title.

Emergency services were called to Holyhead's Newry Beach at about 22:50 GMT on Sunday.
The woman was pronounced dead at the scene. Officers believe she is local to the area, although no formal identification has taken place.
The death is not being treated as suspicious and the coroner as been informed.
Insp Jason Higgins of North Wales Police said: "We are now in the process of locating and informing her family. Our thoughts are with them at this very difficult time"

The female panda Tian Tian came into season late on Wednesday night and she was artificially inseminated in the early hours of Thursday.
Keepers then tried to introduce her to male Yang Guang but her hormone levels had already dropped and he guessed she was not interested.
The zoo will now keep Tian Tian under observation for the three months in the hope that a panda cub will follow.
Edinburgh zoo acquired the pandas on loan from China in 2011 and previous attempts to mate the pair have failed.
Tian Tian, which means Sweetie, and male Yang Guang (Sunshine) were the first giant pandas to live in the UK for 17 years.
The last pandas in the UK, Ming Ming and Bao Bao, left a zoo in London in 1994 after failing to mate.
A gas mains exploded into flames causing homes to "shake" in Whitstable, Kent.
[NEXT_CONCEPT]
The UK's leading bookmakers face paying out more than £10m if Leicester City win the Premier League title.
[NEXT_CONCEPT]
A woman's body has been discovered at a beach on Anglesey, police have said.
[NEXT_CONCEPT]
Edinburgh Zoo's pandas did not manage to mate naturally, it has emerged.