Hi Hoa,
Greetings from Syncfusion.
Currently, we don't have direct support for rendering the chart grid lines above the series, but you can achieve this with the help of plotBand property in the axis. You can render multiple plot bands which will render the line horizontally when setting the plotBand property to the primaryYAxis. Here we have created the method buildPlotBand, it returns the list of PlotBand based on the maximum range of y-axis and interval value and set this list to plotBand property in the primaryYAxis. We have shared the sample below for your reference.
Screenshot:

Regards,
Yuvaraj.
Attachment:
i179542_bb061afb.zip