Even though he hated it, and never got used to jumping from the tree, Gene continued to do so. Copy the line from the story that explains why he does this:

I read this book last year so take this with a grain of salt.

Gene jumps from the tree to compete with Finny due to the fact that Gene is always trying to do better than Finny. He is put under the grasp of peer pressure due to Finny almost forcing him to do this. Ever since he first jumped Finny kept pressuring him to jump since he thought Gene enjoyed jumping.

Rather than uses this answer you should probably just reread chapters 3-4