After Gandalf fought the Balrog he died (they both did). But, Gandalf was returned to Middle-Earth as Gandalf the White (no longer Gandalf the Grey).
Who resurrected Gandalf? Was it Eru? I thought Eru was pretty hands-off but I cannot think of anything else that could do it. If it was Eru do we have any reasoning why this nudge and nothing else?
My impression was that it was Eru, but many say it was Manwë who sent the Wizards on their mission in the first place.
Gandalf said his mission was not done.
So, if Gandalf was still Gandalf but powered up did Eru give him extra juice or was Gandalf nerfed to begin with and Eru just removed the limiters?
(I get I am maybe asking unanswerable questions.)
Gandalf was a lesser Angel, a Maia. He was Olorin in the West, wisest of the Maiar. He was cloak in reduced power and reduced splendor. When Eru sent him back he was far less cloaked and far more powerful.
The dumbest scene in the Movies for ignoring lore, was the Witch King breaking Gandalf’s staff at the Gates of Minas Tirith.
My favorite scene in the entire book was the confrontation between Gandalf and the WK. Jackson blew it…but I forgive him.
To answer the OP: it was Eru. Gandalf speaks cryptically, of course, but he says “I strayed out of thought and time, and wandered far on roads I will not tell.” If he was beyond the scope of Time, that puts him beyond the reach of Manwe and the Valar. The Valar are bound to the physical realm of time and space.
I assumed that it was either Eru or the combined will of the Valar.
I thought everything* in ME was bound inside of the ME universe. All who entered could never leave until (maybe?) the final battle.
Don’t all other spirits return to Valinor?
Is that true after the bending of the horizons?
At the creation, the Valar and Maiar “entered into” Ea, the physical universe, to run things; and stay there until the end. Eru Iluvatar remained above, outside, beyond His creation.
All elves and visiting Maiar return to Valinor – either through death, or on a boat from the Grey Havens. My guess is that if Gandalf had died in battle and was not designated for re-enlistment, he would have gone back to Valinor (the same destination as at the end, on a boat) and resumed his form as Olorin the Maia. But he was called into a conference with the Big Guy, on roads he will not tell.
Pleonast: yes. Even after the bending of the world, Valinor remains in the physical realm. It’s just on a plane that mortals can’t reach. Think of it as a yardstick balanced on a globe … the ships of men sail on the surface of the globe, but the ships of elves can sail on the yardstick.
So, if you die you end up in the Halls of Mandos but if you sail to Valinor then…what? Chill on a beach sipping umbrella drinks?
Most Elves who die don’t remain in the Halls of Mandos forever; they usually are reincarnated after a “time of reflection” in the Halls.
I believe that those who sail to Valinor enjoy an eternal life there; if I remember correctly. That may or may not include the tiny drink umbrellas. 
Elves who sail to Valinor enjoy eternal life. But, then, they had eternal life to begin with. When the last king of Numenor was planning a trip to Valinor, he was warned that he would still be mortal, and that Valinor’s awesomeness might burn him out faster.
In Valinor, the line between the material world and the spirit world is apparently a bit blurry. Elves who have been to Valinor and back live in both worlds simultaneously.
Most of those who sail to Valinor already had eternal life. The 4 exceptions were Bilbo, Frodo, Sam and Gimli. They earned a nice permanent vacation, including tiny umbrellas, healing from their physical and spiritual wounds … but eventually died anyway.
Yeah, Valinor is about as nice as a place can be within the physical world… but it’s still within the physical world. Go there, and you’ll be able to find peace and contentment, but not immortality.
(aside)
Tolkien and C. S. Lewis apparently disagreed on this point. In Lewis’s Space Trilogy, it’s explicitly stated that there are places within the physical world (implied to be on Venus, this being before we learned that Venus was a hellhole) where mortals don’t, in fact, age and die, because those places were never subject to the Fall.
Oh, and to the OP, yeah. Eru very seldom interferes directly and unsubtly in the World, but there’s nobody else who could have done what was described. And He does interact subtly and indirectly, much more often: For instance, Gandalf at one point says something like “Bilbo was meant to find the Ring, and not by its maker, and that is a comforting thought”.
I’ve often vaguely wondered where they buried Frodo and Sam.
Thank you both for correcting my recollection!
I wonder if Elrond phoned Manwë asking if it was cool to bring those four:
Elrond: Manwë! What’s up bro? Listen, I’ve got a few really good guys here who totally did us a solid. Is it cool if I bring them to your pad to chill out for a while? They’re fine crashing on the couch.
Yes, of course. That’s exactly how it went down.
Eru intervened with Gandalf, as has already been said. Eru did similar when Ar-Pharazon invaded Valinor. Implied in HOMES and other writings is that Eru probably intervenes in much smaller and undetected ways regularly.
So he makes these little interventions against poor little 2nd-rate Sauron, but he lets Morgoth run amok over his creation until it gets so bad he has to jump in hip deep, after much pleading and gnashing of teeth. Of course, we’re not supposed to understand the actions of deities, they are so far above us, woooooo.
I just need to remember that this Eru dude is an imaginary stand-in for another imaginary being. That puts it all in perspective for me.
But He didn’t just let Morgoth run amok over his creation. He re-wrote the entire Music to incorporate Morgoth’s dissonance, and to turn it into a more beautiful whole. If we don’t recognize that, it’s just because we each only hear a small part of the whole Song.