Generally speaking, the touchpad scrolling is not explicitly supported in WPF controls. The vertical scrolling works because the scrolling gesture is translated to a MouseWheel event. You can observe the same behavior with a standard Microsoft DataGrid (vertical touchpad scrolling works and horizontal touchpad scrolling does not work).
If you want to enable horizontal touchpad scrolling, you can use the approach demonstrated in the following article: Support Horizontal Scrolling of TouchPad in WPF Application. After you detect a horizontal touchpad scroll, you can use the ChildrenOfType method in order to find the GridViewScrollViewer element inside the RadGridView and use its ScrollToHorizontalOffset method.
I am attaching a sample project demonstrating this approach. Hope you find this helpful.
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