"Gone" doesn't necessarily mean "dead". It means that she's gone missing and the people considering her missing have no information about her whereabouts.
Sure, the word is sometimes used as a synonyme for "dead", it doesn't mean that anyone "gone" is dead.
Though I have no information about Ocean's intentions about Summer's character, IMHO, the story is mainly about MC moving on and dealing with the fact that Summer is gone and likely isn't coming back.
So I highly doubt that we'll see her again in anything but occasional flashbacks.
But, wouldn't it be a crazy end to the story when finally the MC and his LI are together and then, KABOOM, Summer shows up