Yes, in John Knowles' A Separate Peace, Gene fights with Cliff Quackenbush. Quackenbush is the Crew Manager and Gene is the Assistant Crew Manager. Quackenbush resents Gene and mistreats him on the first day of the position. Quackenbush also assumes Gene is in someway maimed, since Gene's taken a lowly position that involves little physical activity even though he's a senior with no hope of someday being Crew Manager. When Quackenbush refers to Gene as maimed, Gene hits him:
I hit him hard across the face. I didn't know why for an instant; it was almost as though I were maimed. Then the realization that there was someone who was flashed over me."
The boys wrestle for a few moments and then fall into the water, which causes them to let go of each other. The implication is that Gene has changed since Phineas was injured. Punching Quackenbush is probably not something he would have done before he caused Phineas to lose his balance on the limb.
In Chapter 6 of A Separate Peace, Gene is involved in a very brief fight with Quackenbush. He has just returned from his break and from visiting the injured Finny and knows that he must follow through on his promise to Finny to get involved in athletics. He halfheartedly fulfills the promise by volunteering to be a gopher for Devon's crew team. Gene shows up late at the crew house, and Quackenbush (the crew manager) chides him for his tardiness. Gene already dislikes Quackenbush because of his unfriendly reputation, and when Quackenbush starts to goad Gene (through his condescending comments), Gene refuses to take it and tells him that he doesn't know anything about him. When Quackenbush gets upset and refers to Gene as "maimed" (79), perhaps referencing Finny's injury or at the least reminding Gene of what he had done to Finny, Gene punches Quackenbush in the face, and the two fall into the river. The "fight" ends with some harsh words but no more physical exchanges.
Gene most likely loses his temper with Quackenbush because of his inner struggle with his guilt. Plus, Quackenbush is the exact opposite of Finny, and as Gene has to deal with the crew leader's surly attitude and comments, it reminds him even more of Finny's absence.
Gene doesn't ever fight anyone exactly as we think of fighting. He does:
- push Finny out of a tree sneakily which puts the story in motion
- participate in the game of blitzball which pits every player against the rest
- get ganged up on by the rest of the guys about the truth of pushing Finny out of the tree not once, but twice (Butt Room, 7; Assembly Room, 11-12)
- fight verbally with Finny in chapter 12...
Hope that helps. The phrasing of your question may have led me off track.