Tracking Net Income and Net GCI can be done in Sisu using the Income report, the Commission Form, and logging your operating expenses.  Here is a video that shows how all this works together.

Video Note: To optimize viewing, watch this video on Loom. You can do this by clicking the square icon with an arrow coming out of it, which appears in the top-right corner of the video.

Also, this article talks more about tracking expenses in Sisu.