Hello Hong, 

I did little research ,looked into API and I dint find any way to remove/edit the unit label from the grid view , although I did find the dispayFormat property to set on timescale to format the display.

Here is the complete online help document with detail explanation:

https://ko.infragistics.com/help/wpf/xamgantt-configuring-the-xamgantt-timescale-display-formats 

And to edit the unit label from the grid view is a new product idea for us which you can suggest for future version at : http://ideas.infragistics.com