Learn Russian: Кто-нибудь зажигал эту свечу и потом гасил её? - Did anyone light this candle and then put it out?

This is a classic example of the "annulled result." Using the perfective зажёг would imply the candle was lit and is still lit right now. Because the candle was lit and then subsequently put out, the result of the lighting was reversed. When a past action's result is undone by the time of speaking, Russian requires the imperfective aspect.

If putting out the candle reversed the lighting, why is гасил also imperfective instead of perfective?

While you could technically use the perfective погасил to emphasize the current unlit state of the candle, using the imperfective гасил here triggers the "general factual" use of the imperfective. The speaker is simply asking about the pure fact of the action occurring in the past ("Did this event take place?"), rather than focusing on the result.

Could I use кто-то instead of кто-нибудь here?

Not without changing the meaning. Кто-нибудь is used in questions for a true unknown ("anybody at all"). If you used кто-то (somebody), the sentence would imply you already know that someone played with the candle, and you are just wondering who that specific, yet unidentified, person is.

Is гасил related to the verb гаснуть?