Single horizontal line showing up on gantt

Below screenshot shows the line that is showing up on the top of my Gantt. I would like to know how to add horizonal lines like this to all tasks to create a grid or remove this line.

Hi Ndmoroz,
I tried to reproduce the Gantt timeline as in the screenshot but the result differs from what you sent. As you can see in the snippet, there is no horizontal line on top:
Can you please provide me some kind of example where I could explore this behavior? You can change the above snippet tool for it (please, reproduce your code, then click the “Share button” and send me the new link). Otherwise, it’s really difficult to tell what may be wrong.