How can I draw or turn off grid lines only for specific columns?

You can remove borders for the specific columns with

   SetStyleRange(CGXRange().SetCols(nFrom, nToCol), CGXStyle()
      .SetBorders(gxBorderRight, CGXPen().SetStyle(PS_NULL))));

You might also override GetStyleRowCol() and call style.SetBorders() to change the border at runtime depending on specific criteria. See “Can I outline cells depending on their value?”.