Force push probably would do the trick
There are several instances in canon of a Force push or Force telekinesis being used to move gases. For example, Aayla Secura, Yoda, and Anakin Skywalker used the Force to repel a torrent of poisonous gas in The Clone Wars (“The Zillo Beast Strikes Back”):
Most notably, in Shattered Empire, Luke Skywalker seemingly was able to use Force telekinesis to stand unharmed (and indeed, seemingly not even blown back) in the middle of a large, fiery explosion:
Note the explosion to both sides of Luke, and the untouched area around him.
Darth Sidious and Darth Vader used a similar technique to deflect an explosion:
Both guards immediately activated and tossed grenades; five seconds
later the tunnel behind them reverberated with the sound of the
explosions, lylek screams, and the rumble of falling stone. The blast
wave roared from the confines of the tunnel. Vader and the Emperor
used the Force to deflect the bulk of the wave from them, but the
power of it drove the two Royal Guards face-first into the floor,
their armor scraping along the stone.
Lords of the Sith
And finally, Kanan Jarrus held back a portion of the flames from an explosion in Star Wars Rebels:
Not being as strong as Luke, he had…a little more difficulty.
There doesn’t seem to be any reason a Force push wouldn’t work on a flamethrower. The usual caveats apply, however: Force telekinesis in combat is harder than in isolation; how well it works depends on the skill and power of the Jedi, and how many other threats they have to deal with at the same time (i.e., it requires concentration). Other options, such as dodging, may be better, more practical, or indeed unavoidable. For example, Mace Windu opted to dodge Jango Fett’s flamethrower:
He faced Jango Fett—and found himself looking down the barrel of a
A burst of flames reached out for the Jedi Master, igniting his
flowing robes. With both Dooku and the bounty hunter so close, and in
such a vulnerable position, Mace just leapt away, lifting himself with
the Force to fly out from the box and land in the arena. He pulled the
burning robe from his back, throwing it aside.
Attack of the Clones
There are even other options
While (short of mere evasion), deflecting a flamethrower stream with a Force push may seem the logical choice for a moderately powerful Jedi, there are other ways of dealing with flamethrowers.
A Force user could opt to use the Force to deflect the gun itself, as Ahsoka Tano did with a blaster in the Clone Wars episode “Wookiee Hunt.” It is possible that, with sufficient power, a Jedi could even conceivably absorb the energy, as Yoda was able to do with Force lightning (and which, rather than Force push, might be what Luke is doing in one of the previous examples). And of course, for a skilled (and fast) enough Jedi, there’s always the obvious option: move the person shooting the fire!