ガスを消すのを忘れたわ。

gas (state of matter, e.g. poison gas, natural gas); gasoline; gas; petrol; dense fog; thick fog; gas stove; gas cooker; gas range; flatulence; gas; wind; fart

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to erase; to delete; to cross out; to turn off power; to extinguish; to put out; to bump off

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

to forget; to leave carelessly; to be forgetful of; to forget about; to forget (an article)

indicates emotion or admiration; indicates emphasis; wah!; boo!