Does this 2x4 support my gable ridge?
You can sister another 2x4 that's turned 90 degrees on it's side to help with the twist (basically make an L with the two boards).  To get them to pull together, first clamp them together and then use some deck screws to attach the two.

It's hard to tell if the twist is from buckling under load or if it's just warped from a moisture change.  And from looking at this on a web site, I think it will be difficult for many of us to say if it's necessary.  (Though even if I were to see it in person, I may be clueless since we outsource all of our roof work to someone with a crane and even they use pre-manufactured roof trusses where every board is load bearing.)

However, it is safe to say that nothing bad will happen if you reinforce it, while it could be disastrous for you to remove it.