Gunnar's and Scobies thoughts on suicide stem from a self-consciousness that Gunnar describes when he says the following to Psycho Loco:
"Might as well kill myself, right? Why give you the satisfaction. The trippy part is that when you think about it, me and America aren’t even enemies. I’m the horse pulling the stagecoach, the donkey in the levee who’s stumbled in the mud and come up lame. You may love me, but I’m tired of thrashing around in the muck and not getting anywhere, so put a nigger out his misery” (226).
Why does everything seem to turn downhill in Boston? I think it's the awareness that the boys have of the underlying racist tendencies that even exists among the backdrop of multiculturalism found in Boston (contrasted sharply with the single cultured settings earlier in the book). Scoby goes ahead and kills himself, leaving Gunner to play the part of Osamu Dezai, "the heavy hearted writer who wandered the back roads of Japan struggling to raise the nerve to commit suicide in the Tamagawa River" (190).
The nihilism in the book seems to peak during the scene when Scoby is asking Gunnar about the height of various buildings in Boston... obviously planning his suicide. Gunnar knows Scoby is going to kill himself. I think he doesn't stop him because Gunnar feels just like Scoby does, the only difference is that--like Osamu Dezai--Gunnar is still gathering the courage needed to kill himself.