The Format / Gridlines option is used for enhancing the visibility of a view, by changing the pattern and colors of gridlines (vertical or horizontal lines that divides columns or rows in a view).
Gridlines can be formatted by performing the following steps:
Click on Gridlines in the Format menu
OR right click on Gant Chart panel select Gridlines
Select the type of gridlines you want to change in the Line to change list
Select the line pattern you want to use in the Type box, under Normal
Select the line color you want to use in the Color box, under Normal
Click the OK button to save these settings
For setting contrasting gridlines at specified intervals, select a line type, pattern, color and interval under At interval
Note: If you don't want a gridline to appear in a view, select the blank area in the Type box.