The gerund in this sentence is "learning". A gerund is a verb form which is acting as a noun, in this case after the preposition "in".
Annabeth's actions in Chapter 18, when she distracts Cerberus with the red ball she stole, prove that she is intelligent and quick on her feet, always preparing. These are all traits of a daughter of the goddess Athena.
I Googled this and it came up with the result of $234,900
Hamlet killed Polonius believing him to be Claudius spying him.
