Gene is an interesting one, because one main point of the novel is that the war impacts him less than his experiences with Finny did. Gene gives an entire analogy at the end of the book where he shows that the war was hard, but that before he ever got to the battlefield, he had fought his own personal war with himself over thre Finny situation. During his time at Devon, Gene had confronted his insecurities with himself, his feelings of jealousy over Finny's popularity and seeming ease in life, and dealt with his own morbid ability to hurt and damage others. The war is so traumatic for people often because they have to face the reality that they are capable of killing and injuring other human beings who they don't really have anything against personally. Gene came to that realization before he ever joined the war; he tried to hurt his friend. He was capable of great evil towards his good friend. In the third to last paragraph of the book, Gene says,
"I never killed anybody and I never developed an intense level of hatred for the enemy. Because my war ended before I ever put on a uniform; I was on active duty all my time at school; I killed my enemy there."
So while other boys had to face that realiy on the battlefield, Gene had already fought with himself over that and come to terms with it. Yes, the war does impact him; for example, it gives him a horrible experience with Leper, it impacts his life at Devon as troops and supplies for the war move onto campus, and it takes many of his friends away from him. However, Gene struggled with the issues that war raised before he ever got to the real war. I hope that those thoughts helped; good luck!