I am not familiar in details of the specific implementation of the Gantt control in the tested application. Though, I used the sample Telerik WPF demo application for testing purposes.
The sample implementation of the discussed control does not support the direct scrolling through a method in the Testing framework. For this I submitted a feature request to our public feedback portal - you can find the item here.
So, the approach you can go for, is to use the visual scrollers on the GanttView control, which I mentioned before - please, see the screenshot attached. The buttons you can use to scroll the content are listed in the app visual tree as separate buttons with unique names. I used the Test Studio tool to automate these and it worked with the UserPress() method invoked on the RepeatButton control.
You can explore the custom application under test and what options there are to automate the scrolling in it. In case you need further assistance, I will appreciate if you can prepare a sample app to demonstrate how the control is used on your end. Since it is quite complex control, it will be nice to know as much details as possible.
Thank you once again for your time and cooperation.