: Gandalf tells Aragorn, Legolas, and Gimli that he can now see many things far off.

:: snip

This is said in the context of saying how his perceptions have changed. The full line is "I can see many things far off, but many things close at hand I cannot see." Which, since you seem determined to take it literally, would argue that he did need to talk to the ents to have news of Fangorn, since it was not just close at hand, it was all around him.

::snip

: 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.

I'm well aware of it. What the heck does it have to do with this discussion?

: "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!
:: snip
You can roll small stones down a mountain forever and not start an avalanche, if the ground is not ready to slide (and I have exhaustively and scientifically tested this point, so no gainsaying, LOL).  The question is, did the Ents get angry on their own, or did Gandalf stir them up? 
:   ...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.
: 

Gandalf is careful to attribute the rising of the Ents to the Ents themselves. The Ents are a proud folk, and if they thought that they were being manipulated by one wizard to move against another, they likely would not move.