What is Gandalf Talking about?

[Armetiel]

In the TTT in the chapter The White Rider, Page 484 Gandalf says to Aragorn, Gimli and Legolas: (this is when he first reunites with them in Fangorn)

The Ring now has passed beyond my help, or the help of any of the Company that set out from Rivendell. Very nearly it was recealed to the Enemy, but it escaped. I had some part in that: for I sat in a high place, and I strove with the Dark Tower; and the Shadow passed.

I can't figure out what Gandalf is talking about here. Is he talking about the part in the FOTR when Frodo puts on the ring to hide from Boromir, because the eye of Sauron almost finds him then, but the shadow passed when Frodo took of the ring, so I don't understand what Gandalf would have done then...and if he's not talking about that episode, then what IS he talking about...I've read the whole LOTR many times, but I can't seem to figure this out lol.