I am not super into the LOTR franchise but, my friend was telling me that Gandalf is super over powered and they don't display it correctly in the movie. His exat words "Gandalf could fart and destroy a whole orc army by farting!" Obviously he was exaggerating a bit, but how powerful is he really?
Not only is he "killable", but he actually was killed.
This is made clear many times in the book.
“The darkness took me, and I strayed out of thought and time, and I wandered far on roads that I will not tell.”
“Naked I was sent back-for a brief time, until my task was done. And naked I lay upon the mountain-top.”
“I tarried there in the ageless time of that land where days bring healing not decay. Healing I found, and I was clothed in white.”
(Book 3, Chapter V - The White Rider)
“I have not passed through fire and death to bandy crooked words with a serving-man till the lightning falls.”
(Book 3, Chapter VI - The King of the Golden Hall)
“Behold, I am not Gandalf the Grey, whom you betrayed. I am Gandalf the White, who has returned from death.”
(Book 3, Chapter X - The Voice of Saruman)
January 25: He (Gandalf) casts down the Balrog, and passes away. His body lies on the peak.
(Appendix B: The Tale of Years)
Gandalf is the Maia Olòrin in Human form.
There is a chapter in Unfinished Tales that gives details on who and what the Istari are.
Emissaries they were from the Lords of the West, the Valar, who still took counsel for the governance of Middle-earth, and when the shadow of Sauron began first to stir again took this means of resisting him. For with the consent of Eru they sent members of their own high order, but clad in bodies as of Men, real and not feigned, but subject to the fears and pains and weariness of earth, able to hunger and thirst and be slain; though because of their noble spirits they did not die, and aged only by the cares and labours of many long years.
Unfinished Tales | The Istari
As Gandalf he is susceptible to death and the weariness of time, in the event of death though his spirit would return to Valinor for healing. His spirit will endure beyond the end of the world.
When he dies fighting the Balrog, Eru (God) intervenes and sends him straight back with a new body. Gandalf the White is a new incarnation.
Gandalf is powerful, but kill able. Olòrin is immortal.