Aerith's last ability checks at the moment it would trigger to see if you gained life this turn. If you didn't, the ability won't trigger at all. Once your end step begins, it's too late to gain life in order to cause this ability to trigger. However, the last part of the ability will check how much life you've gained as the ability resolves, so if you've only gained 5 life this turn by the time the end step comes around, you could still gain 2 more life while this ability is on the stack in order to return the target creature card to the battlefield instead of your hand.