Throughout this book, we really only get Gene's perspective of things, and so we have to guess at the other characters' thoughts and feelings. Up to this point in the novel, we can sense that Gene is incredibly jealous of Finny--so much so that the jealousy is tinged with a bit of bitterness and dislike. Finny is the fun guy, the popular guy, the one that "gets away with everything." And, Ginny is a bit resentful, and definitely jealous. We don't really get Finny's take on their relationship, and he is so laid-back and friendly with everyone that it is hard to separate his feelings for every person he meets as being different from those he has for Gene.
However, as they are strolling on the beach, Gene notices that "people were looking fixedly at [Finny]" He takes a look at Finny to see why, and notes that he radiates health and looks good. But then, here is where Finny jumps in and says,
"Everybody's staring at you...it's because of that movie-start tan you picked up this afternoon...showing off again."
Here, we all of a sudden realize that Finny might be jealous of Gene too! They both see people staring, and automatically assume people are staring at the other person. This reveals how much Finny really does admire Gene also, and that their relationship might be more reciprocal and balanced than previously thought. Finny also remarks almost bitterly, although he says it as a joke, that Gene is "showing off again." So, both boys admire each other, are a bit jealous, and assume that attention and positive affirmation are being given to the other. It's a rare glimpse into Finny's feelings for Gene, and John Knowles uses that attention-filled walk as a tool to reveal it. I hope that those thoughts helped a bit; good luck!