To reproduce: populate RadGanttView with data and hide some of the columns in a button's Click event: 

    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Me.RadGanttView1.GanttViewElement.Columns(0).Visible = False
    End Sub

The column is not hidden until you move the splitter.

Workaround:

    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Me.RadGanttView1.GanttViewElement.Columns(0).Visible = False

        Me.RadGanttView1.GanttViewElement.InvalidateMeasure(True)
        Me.RadGanttView1.GanttViewElement.UpdateLayout()
    End Sub