The effect/eminence/fruit of chanting Bhagavad Gita has been explained by Sri Varaha Swamy to Sri Bhu Mata in Varaha Puranam as Gita Mahatmyam. This needs to be chanted after chanting Bhagavad Gita to obtain fullest blessings.
Arjuna gives up fighting even after submitting to Lord Krishna, so the Lord cheekily says to Arjuna that he is talking like a great scholar while grieving for others when he is not supposed to.
Arjuna realises he is overcome with an uncharacteristic despondency so powerful that he is unable to choose the righteous path that is best for him. So he seeks refuge in Krishna as his Guru to guide him.