Tom Bombadil was a poorly conceived character in the books, so I don't blame them for not including him in the movie.
If Gandalf really gave two shits about saving the realm he would have just given the ring to Bombadil and sent him on the journey.
He is not only immune to the power of the ring, he seems to have his own power over it.
He can wear the ring without being affected by it in any way, he can see Frodo when he puts the ring on and is invisible to everyone else, and when they are staying at his house he does a magic trick and makes the ring disappear then reappear at his whim.
Galdalf even points out afterward that the ring has no power over Tom Bombadil, but then closes the hatch on the idea of giving it to him instead of Frodo by saying that he wouldn't take the ring seriously and would most likely misplace it.
So basically Tolkein wrote a character that made the entire story so far and from then on redundant, and then did a few lines of dialogue in the next book to make sure it wasn't a plot hole.