Why do you think Gene is unable to say he had pushed Finny out of the tree?
There are two reasons, in my opinion, for Gene's inability to admit this, to himself, or to anyone else. First, if he is able to admit this, he must acknowledge some unpleasant qualities in himself, envy, for one, and a feeling of deep inadequacy, another. These are unpleasant truths for anyone. Second, the consequences of his having pushed Finney out of the tree are cataclysmic, a life ruined, the judgement of his peers, and even possible criminal action by authorities. Gene is young and unformed. So, he doesn't realize that the burden he carries in not confessing is even heavier in many ways than the burden of confessing.