Can you clarify your situation? Did you graduate HS in May/June of this year? And are applying to go to college starting in Jan?
If you already graduated you don’t explain anything in the section you highlighted in your OP.
But, you do need to explain your gap semester in the area of the common app that asks if you took a gap year (yes, explain it even if you are only taking a GAP semester.)
It would also be helpful if you HS counselor addresses your reasons for a gap semester in their LoR/report.