Question

you left the gas on, did'nt you? (end ----did you?)

Answer 1

you left the gas on,did you?

Answer 2

$\underline {\underline {\Huge \mathfrak{Answer ;}}}$

You didn't leave the gas on, did you?

___________________________

Here, 'did you' is a positive question tag, so we need to write 'didn't leave' in negative in the first part of the sentence.

Hope it helps!