Gandalf tells Aragorn, Legolas, and Gimli that he can now see many things far off. Don't forget that he sat in a high place and struggled with Sauron in thought while Frodo was wearing the One Ring on Amon Hen. "Even as we talk here, I hear the first rumblings," he says, sitting with the others on the hill (or near it) where Treebeard had met Merry and Pippin a couple of days before.

Gandalf had no need to talk to the Ents as far as gathering information goes. But I think he attributes their awakening fully to the hobbits.

  'You have not said all that you know or guess, Aragorn my
  friend,' he said quietly. 'Poor Boromir!  I could not see what
  happened to him.  It was a sore trial for such a man: a
  warrior, and a lord of men.  Galadriel told me that he was in
  peril.  But he escaped in the end.  I am glad.  It was not in
  vain that the young hobbits came with us, if only for Boromir's
  sake.  But that is not the only part they have to play.  They
  were brought to Fangorn, and their coming was like the falling
  of small stones that starts an avalanche in the mountains.
  Even as we talk here, I hear the first rumblings.  Saruman had
  best not be caught away from home when the dam bursts!

  ...But now [Treebeard's] long slow wrath is brimming over,
  and all the forest is filled with it: it will soon be running
  like a flood; but its tide is turned against Saruman and the
  axes of Isengard.  A thing is about to happen which has not
  happened since the Elder Days: the Ents are going to wake up
  and find that they are strong.

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