What is a suitable theme in Chapter 6 when Gatsby joins Cody on the five year voyage?
A literary archetype displayed through that chapter is the quest. Every hero goes through one of these and it is the process of the journey that makes him the well-rounded individual he eventually becomes. During this time the character may also acquire a new identity or undergo a transformation. This "quest" might be a possible theme. The idea of transformation might work. It seems that his time with Cody significantly shaped him because he seems to have more images of Cody than his own father (at least one), and Cody is significant enough that both Gatsby the character brings him up and Fitzgerald, the author brings him up. The real life of Gatsby as reported through other characters throughout most of the book seems to be significantly impacted by Cody too.
The image I always get of Gatsby is a type of nautical man well before the discussion of Cody comes around. This might be something the author has crafted in his description from chapters 1-5 so that he can bring the evidence of Gatsby's roots to the surface.
This sequence of events in chapter 6 also demonstrates a longing for identity.