In general, it is college courses taken after high school graduation that cause a student to be treated as a transfer rather than frosh applicant. While some colleges may allow a frosh applicant to have a limited number of credits of college enrollment after high school graduation, many others do not, so if a gap year student wants to preserve frosh applicant status at all possible colleges, s/he needs to avoid taking any college courses after high school graduation.
Obviously, with respect to the OP and deferred entry to Amherst, the OP needs to follow whatever Amherst says, or ask Amherst directly if it is not clear.