This isn't a feature we support out of the box.
However, you could use SChartAnnotations to mimic Major gridlines by calling "+verticalBandAtPosition:andMaxX:withXAxis:andYAxis:withColor:".
The same can be said for minor gridlines, they aren't something we support but could be done by using SChartAnnotations.
More information on SChartAnnotations can be found HERE.
Let me know if you have any other questions.