From the American Gas Association monthly - Volume 23 (1941):
Even Hollywood has been using, for some time now, the expression "Now you're cooking with gas" to denote perfection.
That might be so in the US, where the car fuel we call "petrol" is called "gas", but not here in the UK. I understood, "Ni kuiras per gaso" to mean "We cook by gas, not electricity, a log-burning stove, ktp" The Esperanto for petrol/gas is "benzino"
Funny thing, that's exactly how the saying started out as in America as well, where we call the "natural gas" we use for cooking "gas"...which is tergaso in Esperanto.
If only I had thought to include links to articles that explains that it originally
... alludes to gas stoves, which began to replace slower wood-burning stoves about 1915.
...and then went on to pick up a new slang meaning.