The final scene takes place 500 years after FFVII, suggesting that it takes place 498 years after Advent Children. As to the significance of the scene, FFVII's very ambiguous ending required some sort of definite conclusion. While the scene, taking place so long after the game, leaves much unanswered, it does point to the fact that Gaia survived. Given that Bugenhagen doubted the survival of the Planet regardless of Cloud and company's actions...

Bugenhagen: Ho Ho Hoooo. No Nanaki. You can't stand on your own yet. To do that
now would destroy you in the long run. Reaching up into the
heavens, threatening to snatch the very stars from the great city of Midgar. You
seen it, haven't you? Well, that's a bad example. Looking up too much makes you
lose perspective. When it's time for this planet to die, you'll understand that
you know absolutely nothing.

Cloud: ......When the planet dies?

Bugenhagen: Ho Ho Hoooo. It may be tomorrow, or 100 years from now... But it's
not long off.
Later, after the battle with Gi Nattak and the revelation of Seto's true actions...

Bugenhagen: Listen, Nanaki. Cloud says they are trying to save the planet.
Honestly, I don't think it can be done. For even if they stop every reactor on
the planet, it's only going to postpone the inevitable. Even if they stop
Sephiroth, everything will perish. But, Nanaki. I've been thinking lately. I've
been thinking if there was anything WE could do, as a part of the planet,
something to help a planet already in misery... No matter what happens, isn't it
important to try? Am I just wishing against fate? I am too old to do something
about it... This year, you must go with them! For my sake.
...this aspect of the ending ultimately confirmed the success of the party's efforts to save Gaia, even if their own eventual fates remain unknown to the gamer.