Thanks for using Syncfusion products.
The vertical grid lines visibility can be turned off by setting the value of showMajorGridLines to false.
_chart.primaryAxis.showMajorGridLines = false;
Still, you could change the color when the gird line is visible by using below code snippet.
_chart.primaryAxis.majorGridLineStyle.lineColor = [UIColor redColor];
For more details for Axis customization, refer the below online documentation.
Please let us know if you need further assistance.