Write little things that you did which mattered the most to you. When you say, “I have not done anything special”, I dont think colleges want to hear from you how you invented a machine- that is yet to be launched. Nothing big- just write how you kept yourself busy. Was it by cooking food at times, playing football, visiting friends, making future plans, or anything random you actually did.
Things you may have done in gap year may not be special for colleges, but colleges ask you to mention things you did so that they understand what is “special” for you. What you do in the gap year is what you like the most. It does not have to be academically connected.
Follow back if there are any confusions.