Gene's comment is an acknowledgement of Finny's unique capacity to create his own reality, separate from the real world. The reality Finny creates reflects the way things would be in an ideal world.
During the summer session, Finny exhibited an amazing ability to get away with things, talking his way glibly out of facing the consequences of his actions when he would break the rules. An example of this is in Chapter 2, when he is called to task for using the Devon School tie as a belt. Finny concocts a ridiculous but earnest explanation as to why he chose to do this, and Mr. Patch-Withers, who is ordinarily stern and dour, uncharacteristically breaks into laughter, and allows Finny to get away with his infraction. Finny is able to get beneath Mr. Patch-Withers' forbidding exterior because the world he lives in, a world of innocence and wonder, is so far from the dark, drab reality of the 1940s. Finny's world is steeped in an ingenuousness that is missing in the real world; it represents things not the way they are, but the way that they should be.
In Chapter 8, Finny realizes that his injury will prevent him from taking part in the defining experience of his age - the War. He then purposefully creates an alternate reality, where there is no war. Like his comment about love, Gene realizes that Finny's made-up world is not real, but still, it holds a certain allure - in an ideal universe, there would be no war. With his comment, Gene is acknowledging that he knows that Finny's version of things is not real but that it should be. In addition, he is admitting that Finny's world is so alluring that he, Gene, gives himself up to it, even though he knows that it is not true (Chapter 8).