Hide the center segment of the grid lines to create a gap. 
-  Open a view that displays grid lines. 
- Select a grid line, and click Modify | Grids tab Properties panel Properties panel  (Type Properties). (Type Properties).
- In the Type Properties dialog, do the following: - For Center Segment, select None. 
- For End Segment Weight, End Segment Color, and End Segment Pattern, specify the line weight, color, and pattern of the segments to display on each end of the grid line. 
- For End Segments Length, enter the length of the segments (in paper space) to display on each end of the grid line. 
- Use the other parameters to indicate which grid bubble to use and where it should display. See Grid Type Properties. 
 
- Click OK. Revit updates all grid lines of this type in all views.