My problem is that i have this column that is called Terminated By

A task can be terminated by an employee OR a crew for example. which are both GUID. 

 So how can I achieve to show the employee name or a crew name in that column ..... instead of the GUID.  I also have another column that enable to specify by what type of operator has terminated the task, an employee or a crew, and then when going to the Terminated By column, it opens a combobox of a list of values of employee or crew depending on your choice.

Is this possible?

Yes, a ValueList allows you to specify a DataValue and a DisplayText for each item. This is expressly for the purpose of storing a value and displaying more user-friendly text to the user. 

 But when I set the value into the cell with a GUID, and populate the valuelist, will the grid show the display name and not the value(guid) automatically?

There are many ways to do that.

Can you make a list of your guids and display value for each of them? If that's the case, I think using a value list is the simplest approach.

var list = grid.DisplayLayout.ValueLists.Add();

list.ValueListItems.Add(value, name);

column.ValueList = list