Word was brought to him in Chapter 20, which is on the Nirnaeth Arnoediad.
Where? He knew about the battle, but not because Fingon sent word to him. When Turgon's hosts arrive, they are "unsummoned and unlooked for," so I assume he learned that there was going to be a fight from reports from the eagles.
It says the following:
It is explicit that Turgon was responding to no summons. What is less clear is how he found out at all. Since we know that he didn't let his people out, and none of the Elves outside knew how to get in, I'd say that @Rhiannon's suggestion is the only real possibility, barring Turgon quietly sitting on a palantir, or a suspiciously specific dream from Ulmo.But now a cry went up, passing up the wind from the south from vale to vale, and Elves and Men lifted their voices in wonder and joy. For unsummoned and unlooked for Turgon had opened the leaguer of Gondolin, and was come with an army ten thousand strong, with bright mail and long swords and spears like a forest. Then when Fingon heard afar the great trumpet of Turgon his brother, the shadow passed and his heart was uplifted, and he shouted aloud: 'Utúlie'n aurë! Aiya Eldalië ar Atanatári, utúlie'n aurë! The day has come! Behold, people of the Eldar and Fathers of Men, the day has come!' And all those who heard his great voice echo in the hills answered crying: 'Auta i lómë! The night is passing!'