"Mars Gaiden" it's a collection of three stories by Fuyumi Souryo.
The first, "Namae no nai uma", it's a "Mars" prequel telling the first meeting between Rei and Tatsuya and the beginning of their friendships. Moreover, they're also presented other main characters as Kira and Harumi.
The second and the third stories, "Nemuru Raion" and "1 Karatto no Kajitsu", are oneshots not related to "Mars".
The first tells the strange relationship between Isomu, a guy who pretends to don't have any fear, and Ayame, a girl with particular behaviours; at the contrary, the second tells about Nina, who has some family's problems, and Rui, her classmate who is interested in a close friends of the girl. But what if he's interested in Nina?