Part of Gene's life was taken away when Finny got hurt. This means a few things. The most important thing it did to Gene was made him act very different then he usually would. These changes are very subtle and as the book progresses these changes are shown more and more. These changes continue until Finny's tragic death where Gene realizes the harm he has done.
One of the earliest signs of Gene's change in personality is shown very early in the book. It starts the next semester of school. During the next semester, Gene chooses not to do a sport as most might think a senior would do. Instead he manages a crew team. During his first day he is insulted by his classmates for not doing a sport and they begin to talk about Finny. Gene becomes angry and attacks his fellow students. Before Finny was hurt he kept his anger and thoughts to himself and never shared them with anyone.
This is one of the first signs that Finny's death affected Genes attitude and the way he thinks.
Another appearance of his change in personality happens while talking with his classmate named Brinker. Earlier in the book Finny and Gene did not wish to go into the army. But when Brinker decides to enlist he asks Gene to also. Gene decides he will and if it was not for Finny's return the next day Gene would have gone to the army. This also shows how Finny's injury changed the way Gene acts.
A third example of this happens after Finny had returned to the Devon School. Once Finny was back he decided to jokingly train Gene for the Olympics. Gene agrees to train because Finny was very athletic and by training him, he is filling in the part of Finny that Finny can't do anymore. This is also very odd because Gene is not the type to waste time running when he could be studying. This also proves that Finny's loss made Gene act differently.
These examples grow steadily until it is very obvious that Gene has changed. They show how much Gene was dependent on Finny by the fact that when Finny left his entire lifestyle and personality changed. These are why a part of Gene was taken away when Finny was injured.