Gandalf can't be in The Rings of Power because the show is set in the Second Age of Middle-earth, but Gandalf (as one of the Istari/Wizards) doesn't arrive in Middle-earth until the Third Age, centuries later, to specifically counter a resurgent Sauron, according to Tolkien's timeline. His role was to guide Elves and Men, not to wield overwhelming power or interfere directly in the major conflicts that predate his arrival, which is why he appears in ancient, humble forms.
Because between season 1 and 2 they got the license to use Gandalf. Prior to that he very literally wasn't a decided on character. He would have been either a new character or continued to be one written very vaguely as they continued to try and get the rights had they not gotten them.
Even at his fullest, Olorin/Gandalf was not the equal of Mairon/Sauron, not even close. Same goes for the Balrogs, they were described as lesser spirits while Sauron was described as a greater spirit amongst those that were corrupted by Melkor.
It's not the years; it's the mileage. Gandalf isn't a Maia in the same way Sauron is. He was sent to middle earth as an old man, so his body is much more real than Sauron's “raiment”. He can't use his power to the same extent, he can't change his forms at will.
Gandalf cannot touch or accept the Ring because it embodies Sauron's will and corrupting power, is morally incompatible with his mission, would likely transform him into a tyrant, and would make Middle-earth vastly more vulnerable by attracting Sauron's focus.
It's possible that Gollum's prolonged search for the ring allowed it to maintain a significant power over him, suspending his aging process. In contrast, Bilbo's exposure to the ring's influence waned after he willingly (mostly) relinquished it, allowing his aging to resume more noticeably.
As a Maia, Gandalf was an angelic being in human form, in service to the Creator (Eru Ilúvatar) and the Creator's 'Secret Fire'. He took on the specific form of an old man as a sign of his humility. His role was to advise but never to attempt to match Sauron's strength.
Gandalf's spirit is estimated to be about 24,000 years old (by his own admission) and Sauron is likely the same age. Galadriel is at most 8,000 years of age. Galadriel was an elf, born in Arda.
Frodo, Sam, and Gimli eventually died, even there. They are lands of peace, which Frodo needed in order to properly heal. What their days there looked like, it is unclear. But they did not gain the immortality of the Elves or the Valar.
The Ent who figures most prominently in the book is Treebeard, who is called the oldest creature in Middle-earth.
His task was to do the bidding of his masters and so when he proved unfaithful in doing that task his masters stripped him of his robes and gave them to one who would do their bidding. Gandalf is now the White, Saruman as he should have been.
Sauron was originally a Maia of Aulë the Smith; his name then was Mairon, meaning "the admirable." He learned much from Aulë in the ways of forging and crafting, becoming a great craftsman, and mighty in the lore of Aulë's people.
Gandalf is not permitted to use his full power during the 3rd Age (some exception apply). He has been sent to guide humans to the 4th age, the age of mortals. He can perform minor magic as needed, but he cannot "bust out."
Tolkien's description of Gollum conforms to a Catholic – and Thomistic – account of envy, which is a “sadness of the soul”; and it is Gollum's unbearable sadness and his unquenchable desire for the one Ring that marks his character.
Aunt Google would tell you that his very first (and original ) name as a Maia is Olorin, The elves called him Mithrandir (Grey Pilgrim), Men named him Gandalf (the Grey, later the White), Dwarves knew him also under Tharkun, and in the deep South of ME he was also known as Incanus.
Tolkien's Legolas is extremely introverted. He just doesn't have much to say directly to anyone, perhaps because he's thousands of years old. His odd relationship with Gimli draws him out in a way the hobbits never do, and that's mostly after Frodo and Sam leave the Fellowship.
'The Fellowship of the Ring' (2001)
If Gandalf had stayed dead after his sacrifice in The Fellowship of the Ring, then his demise would be the saddest in the trilogy, but since he came back, the saddest single death scene of all three movies goes to Boromir's.
Bilbo and Frodo are completely worn out from the weight of bearing the ring. And once the ring is destroyed it stops preserving the life of those who bore it. Traveling to the Undying Lands won't grant them immortality, but it will give them a paradise-like place to rest for the last years of their lives.
His two most salient powers were his knowledge and his voice. Saruman was, in a very real way, the junior brother of Sauron both having been Maia of Aulë, Valar of Smithcraft, in the very beginnings of time.
Olorin in Gandalf's body is about 2000 yo, that is when the Istari (wizards) took their physical forms when arriving in Middle-earth. Gandalf's reference to "the lives of 300 men" doesn't necessarily mean he's that exact age—it's more poetic, speaking to his immense wisdom and experience.
Gandalf and Galadriel share a profound but purely platonic relationship. Their connection likely started in Valinor, as Gandalf, in his original form Olorin, often visited the Gardens of Lorien, where Galadriel might have been.
Saruman first appeared during a fourth phase of writing in a rough narrative outline dated August 1940. Intended to account for Gandalf's absence, it describes how a wizard titled "Saramond the White" or "Saramund the Grey", who has fallen under the influence of Sauron, lures Gandalf to his stronghold and traps him.
Sauron's rise to power in the Second Age is portrayed in the Amazon prequel series The Lord of the Rings: The Rings of Power. He first appears disguised as the non-canonical human character Halbrand, and then in the second season as Annatar (a canonical alias of Sauron), both played by Charlie Vickers.