Yes split the wall (right Click a wall > click +Wall > Choose Split wall then hove rover the wall and left click to split the wall in one or multiple locations. Then go back and select the wall and change the height and choose "gable" in the construction mode, you will need to type in the pitch or ratio of the gable before you click submit. Here is a video showing how to do it: https://youtu.be/1lBO7Xj_hnQ?t=334
The link goes directly to split wall yet I suggest you watch the entire video as it will help you out a lot
In the future when you draw the wall it is best if you actually left click to define the different height, then go back and change the height of the wall after either completing a loop or double-clicking. You will find that all CAD programs including Revit And ArchiCad work like this.
I hope that helps you out.