So far, the total number of episodes to be written is unknown. However, there will be several story arcs in Attack on Titan: Gale. This page lists them, plus the episodes.
Note: The Pilot episode aired at June 17th, 2014. However, it is currently not in any story arc. You can read it here: Pilot (AoT:G)
The Retreat arc
The Awaiting Road of Hope NickFusi0n
June 23, 2014
After Wall Maria falls at the hands of the Colossal and the Armored Titans, Patricia Deller retreats to Karanese District with her mother after losing her father. There she meets Nick and Sci, who she starts to treat as true best friends.
The Decision NickFusi0n
June 28, 2014
After witnessing the Survey Corps' return from an expedition, Nick, Sci and Patricia make their decision for the future.
Arrival of the Trainees arc
Enter: The Military