To make a gable roof, select the Roof icon from the Architecture ribbon.
Once in the Roof Sketcher mode, either select the Pick tool and click on a room, to create a roof whose base takes the room's form; or, use the line tools to manually draw the roof base's outline.
Under Roof Edge Settings;
For the pair of opposite edges that are subtending a slope, define the desired Slope Angle (degrees) in the corresponding field and set the Roof Edge Visibility to 'Visible'.
For the other pair of opposite edges , define the Slope Angle (degrees) to 90, and set the Roof Edge Visibility to 'Invisible'.
Click on Apply to apply changes.
Then, to attach the walls to the roof, select Attach Wall tool in the Architecture ribbon, select the desired walls and then the corresponding roof to which you wish to attach them.