Gunblade wasn't a real weapon after all, it was created by Ulti (Ulti>Squall's mind)
In general, Griever Ring, Gunblade, his clothes/necklace/silver belt are very similar to each other.
In the game, one of his equipment - Griever Ring was seen created because he was very scared of the concept in the ring, so Ulti used his feeling to create it to balance everything out because she gave her that sword. The reason why she made GR from his mind, it's because of game-playing purposes. Such game-playing purposes have nothing to do with the entire story.
In FMV timelines, Squall always had used gunblade to kill Ulti. But it's different in real time... again because of game-playing purposes since you rather equip a better sword in order to be stronger.
How Gunblade was created? Squall never became scared of the sword, he focused on it all the time in his mind through the entire story because he wasn't supposed to be scared of it. He needed it to beat Ulti. Ulti knew about that all along, she made the gunblade and left it somewhere in the early Present timeline, somewhere Squall can find it, all the power lie in that sword, to give Squall an advantage so he can beat Ulti easier.
What's the point if the current being of Uli always live forever? That current being won't want to live forever, she/he has to die in order to have peace and leave the power to someone else, in this case, Rinoa. Her wanting to die, that's the whole reason why Ulti made handy stuff for him and his party members so they can beat her even easier.
Do you know how Squall and his party members have met again? When they met again and realised that they remember each other from the past (in about mid of the game) then they started having a feeling - that they never knew how it all started, how, why did they meet each other again, how come they didn't remember. What's the real reason? They couldn't even remember thier past, such past that they have met before (until the end). I know how, it's all because of the current Ulti being's plan. She controlled each mind, made changes in the timelines so all of them can meet each other again. That was always part of Ulti's part because she wanted them to kill her eventually.
That plan is completed. She has left her powers. Rinoa is the most logical choice to be next Ulti in the future because of her mixed emotions, etc. Her knight was Squall. Seifer was Edea's knight, etc.