When Gene visits the tree, what does it resemble?

The tree is a very important symbol in A Separate Peace. For Gene, it represents many different things, such as the need for self-discipline and overcoming your innermost fears. He associates it with his schooldays at Devon when his roommate Finny egged him on to make a daring leap from the tree into the river. Initially, the tree presents quite a forbidding spectacle to Gene; it's tall, majestic, a "steely black steeple." But when he returns as an adult to Devon School fifteen years later, the tree takes on a completely different appearance. Now it seems shriveled and weak, much smaller than he remembers it. In the words of Gene, it seems to resemble

[T]hose men, the giants of your childhood, whom you encounter years later and find that they are not merely smaller in relation to your growth, but that they are absolutely smaller, shrunken by age. In this double demotion the old giants have become pigmies while you were looking the other way.

Gene reflects on how fearful he was when he was a schoolboy at Devon, and he deliberately seeks out those places on campus he always associated with that fear, one of which is the tree. But it no longer holds any such fears for Gene. In actual fact, the tree hasn't really changed all that much but he has. He's gone through so many experiences, most notably during the war, in which he's had to show courage that the tree now symbolizes not so much fear, but the quiet, bookish young schoolboy who didn't really understand its true meaning.