who or what would you say has the greatest control over gawain's fate ?

I would say the Aurthereon code of the knights of the round table probably had most to do with it. Gawain knew he could not let the king fight this battle (part of the whole code thing) so he volunteers. Gawain's subsequent actions are then dictated by this Knight's honour code which, in the end, saves his life.

Sorry the word is spelled "Arthurian". lf you need this in greater detail just specify what you need. You can E mail back on this same message.