I'm back home now. So I have my books available; though not as impressive a shelf as that.
So a fairly straightforward question.
As far as I know, Gandalf only ever "despatched" one evil creature. (Later episodes of The Rings of Power, and other books I haven't read (*), which are possibly on Py's shelf, may prove me wrong.)
If, as a boy, Faramir had been carelessly browsing through old texts in the the Minas Tirith Library, why might he have thought, possibly mistakenly, that Grandpa had beaten Gandalf in that regard.
(Please ignore awkward questions of time, like when he was a boy and when the 2 Gs above did their stuff)
Evidence would be appreciated.
(*) or your better memory than mine)