All the details are here!
A short promotional video was shown at the end of the reairing of episode 13:
Genjuurou: "A second season has been greenlit...?!"
Text: "The days of those heated battles...will once again!"
Text: "TV Broadcast to begin this July"
Text: "Farewell, Gungnir"
As expected, it will begin airing this July (exact date is still TBA). The title will be "Symphogear G: In the distance, that day, when the star became music..." the subtitle is taken from the lyrics of Zwei Wing's "Gyakko no Flugel."
You can check out the official website here: http://symphogear-g.com/
There are new gear designs for Hibiki, Chris and Tsubasa:
The director and character designer from season 1 have left, so some new people will be taking over.
You can read more of the staff details in the Anime News Network article.