If you remember, Frodo seemed to hear Gandalf's voice when he sat on Amon Hen in the scene you mentioned. At that time, Gandalf had just recently been brought back to life as Gandalf the White, and was either in Lórien, or, as he implies, sat on Gwaihir. Either way, he perceived the danger to the Ring, which was still relatively near, and helped the way he could.
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