The expression “to cook with gas”, at the time this is written, is classed as slang.
It is included here because almost every other phrase in this book was, at one time, slang and eventually passed into literary use.
Perhaps this will too. Currently it is used to mean to be strictly up to date, or even a little ahead of the procession, not out of date, as cooking with wood or coal would imply.
Just why the expression should imply ultramodernity is not clear, other than that the use of gas is supposed to make a faster, hotter fire than wood or coal; but electric current as used in stoves is more modern than is gas.