I'm not sure which specific story Gandalf is telling as there are many in the Tolkien's works. Maybe it's about the history of Middle - earth, like the battles against the dark forces or the tales of the rings.
Gandalf originated from a higher spiritual plane. He was dispatched to Middle-earth with a purpose. His background involves being part of a divine order and having a responsibility to bring hope and resistance against evil.
One of the great stories related to Gandalf is his role in 'The Lord of the Rings'. He guided Frodo on his journey to destroy the One Ring. He used his magic and wisdom to protect the Fellowship from various perils. For example, in Moria, he fought the Balrog to buy time for the others to escape.
Since it's not a part of the original Tolkien concept, it might be a fan - made idea. Fans sometimes like to create alternate versions of characters, and perhaps someone thought it would be interesting to have a 'dark' Gandalf.
It's hard to say. Gandalf has a lot of raw power and experience. He's faced some really tough foes in Middle - earth. But Harry Potter is no slouch either. He's got his own set of powerful spells and the ability to think on his feet. However, I think Gandalf might have the edge. His magic is more of an all - encompassing force, and he's had centuries to master it.
Well, first off, Gandalf being a Maiar is a huge part of his origin. It gives him his power and wisdom. Then, the fact that he was sent to Middle - earth by the Valar is also crucial. This mission from the Valar shaped his role in Middle - earth. He was part of a group of Istari, and this also stems from his origin story as he was chosen for this role because of his nature as a Maiar.
Maybe Gandalf appears in the wizarding world of Harry Potter and helps him to defeat Voldemort. He could use his powerful magic that is different from what they have in Hogwarts.
One common theme could be the struggle between their different natures and responsibilities. Gandalf is a wizard with a mission, and Galadriel is a powerful elf - queen. Their love might be in conflict with their duties. Another theme could be the long - lasting nature of their relationship, perhaps spanning centuries or even millennia.
The fact that Gandalf is from the Ainur is a very important part of his origin story. Also, his transformation into the form of an old man is significant. It allowed him to interact more easily with the races of Middle - earth, like humans, dwarves, and elves. And of course, his being sent to oppose Sauron is a central aspect of his origin. It shaped his entire journey in Middle - earth.