In what ways can taking a gap year be harmful to an applicant?
If you take a year off from going to school, colleges want to see that you either had a good reason, or that you have something to show from your unstructured time.
If you did nothing for an entire year and simply enjoyed not being in school or went on vacation, colleges will not be impressed. This is not to say you cannot enjoy your time not spent in school, but a year away from school should still be a year of personal growth and productivity. Spending a month in Spain will not harm you, especially if you volunteered there, stayed in a homestay and learned the language or even just vacationed but spent the remaining eleven months, volunteering, working, or taking classes, but spending a month on vacation and then doing the same for the other eleven will not help you get into college.