I took a gap year after my high school. It technically wasn't a gap year since I reappeared for my 12th grade(senior year) examination to increase my score. In 12th grade, I scored 62% in 2012 and then I improved it to 79% in 2013. The main reason I went for the gap year was to explore more about the 2 engineering fields I was confused about going into (CS and aeronautical engineering). During the gap year, I explored both the fields and decided to go with CS. Now, I've graduated. I'm applying to schools in the US for masters. Should I mention about the gap year in my SOP? If yes, what reason should I mention? Are universities very serious about this stuff?
First, make your SoP forward looking, not backward. Only bring up past actions to support the idea that you are an excellent candidate who is well prepared and motivated and has excellent prospects for success. Your CV explains you past. Your SoP is for the future.
But I see no need to mention anywhere that you have a gap. Your transcripts and record will probably indicate something like that, so it isn't a question of deceit or hiding anything. It is (a) just too long ago to be very relevant and (b) too easily explained if anyone should ask about it.
Your record is what it is and you can't change it. Your future is your own to choose. Put the emphasis in the SoP on forward looking things and why they are achievable. Don't, in particular, apologize for anything in an SoP.
And, finally, I doubt most would notice the gap and fewer would care. Especially if you had some reasons - even a year in the South of France.