As I mentioned in the title, the GridLine is not displayed in the Helixtoolkit MVVM instance. I couldn't find the source of the problem. My application is as follows.
<HelixToolkit:HelixViewport3D ItemsSource="{Binding Objects}">
<HelixToolkit:GridLinesVisual3D Width="600"
Length="600"
Thickness="0.1"
MinorDistance="5"
MajorDistance="10"
Fill="Black"
Visible="True" />
</HelixToolkit:HelixViewport3D>