You should avoid mentioning any low marks or shortcomings about yourself in any of the application documents, including SOP. There are very few universities that ask you about your gap year. For them, you can mention the reason for the gap between your studies. Generally, no university asks about low grades during your study period as your selection depends on various criteria like exam scores, student profile, financials, and so on. Thus, it is advised against mentioning any flaws or low marks.