I want to know the effect on the bonds when the net income increase

I am not really sure what you are asking. Your title and your post seem to be saying two different things.

Net income does not affect bonds at all, so your actual post has no answer.

Your title seems to be more asking what the gain does. Gains, like revenues, will increase net income. And that means any kind of gain and doesn't have to be related to bonds at all. (In other words, the fact that it's a bond is irrelevant.)