In what is listed as Chapters 21 and 22 of Volume 2 (in the edition available on the Gutenberg Project) Gawain fights Launcelot twice. Both times, he loses but Launcelot refuses to kill him.
In both cases, Gawain is very angry. He calls Launcelot a "traitor knight" and dares him to come back and fight to the death. However, Launcelot will not kill him. This is because, as Launcelot says, he is not willing to kill someone who is defenseless. Since Launcelot made Gawain helpless both times, he (Launcelot) does not want to finish him off when he cannot defend himself.