(NOTE: the following is the pre-New 52 origin)
Among the many mistakes they've made in their millenia of existence, making Guy Gardner a Green Lantern is probably one of them. The by-product of an abusive childhood, Guy managed to have a promising career as a football player in college until an accident forced him to quit. Landing a job as a caseworker for a prison, Guy made friends with one of his charges and future fellow GL, Hal Jordan. After working as a special needs gym teacher, Guy was one of the two men chosen to be the new Green Lantern of Sector 2814, but since Hal was closer, he got Abin Sur's ring, with Guy being his back up. A series of later incidents altered Guy's personality, turning him into a rude, brash, obnoxious, aggressive, arrogant jerk who somehow managed to become a founding member of the Justice League International. Though most of his "friends" (they use the term lightly) find him to be a regular jackass, Guy has also shown to have the soul of hero and has proven his worth time and again....when he's not arguing with his fellow GL's, or getting decked by Batman.
Guy Gardner has past training as a criminal and football player.
During his time as Warrior, Guy's Vuldarian DNA gave him a formidable set of powers: Superhuman strength, shapeshifting, organic weapon production (the ability to grow weapons from your body), energy projection and absorption, and a regenerative healing process