First things first- check out the colleges you plan on/ hope to apply to and find out where they stand on
gap years. Harvard is very supportive for instance, but as you known the chances of admissions there are miniscule at best. A Call to the admissions office or a conversation with the Admissions counselor for you area might be the best way to find out. Some may be totally supportive, others may not encourage gap years.
PUBLIC U’s or colleges may not allow gap years.
So do some research.
You wont be rescinded if you change your plans because a commitment to taking a gap year is not a requirement for admission.
I would hesitate to mention your gap year plans in your application.
But instead I would let you passion and commitment for the field shine through in your essay.