There are a couple of reasons why the clothing comforts Gene. First, he is far more humble and quiet than Finny is, and wishes he sometimes exuded the confidence and waggish ways of Finny himself. By assuming the clothing, Gene feels that he more easily possesses those traits he finds admirable in Finny.
Also, it is comforting to Gene to wear the clothing of Finny since he still feels responsible for the accident on the tree branch. It seems a bit like a way of honoring him, and that is why he finds contentment in the clothing choice.