A side story manga set in the Cosmic Era timeline that takes place concurrently with Mobile Suit Gundam SEED.
It is one part of the first wave of the SEED Astray multimedia project.
Whereas the main story of Gundam SEED focuses on the struggles between the Earth Alliance, ZAFT, and Orb Union, the Astray series is focused on the three MBF-P mobile suit prototypes and their respective pilots and organizations, who mainly exist on the outskirts of the main SEED storyline.

The MBF-P02 Gundam Astray Red Frame is piloted by Lowe Guele of the Junk Guild, and the MBF-P03 Gundam Astray Blue Frame is piloted by Murakumo Gai of the mercenary group Serpent Tail.
The third Astray, MBF-P01 Gundam Astray Gold Frame, is piloted by the primary antagonist of the Astray story, Orb nobleman Rondo Gina Sahaku, and later by his twin sister Rondo Mina Sahaku.
[Via Gundam Wiki]

Notes:

• The Re:Master edition includes ASTRAY and X ASTRAY with the extra chapters moved to different spots.