Hi.

Incase you are willing to take admission in a desired course next year, then you can prepare for its entrance examinations so that you can achieve the best possible results next year.

Besides that, you can learn from various online courses depending upon your interest. NPTEL is an online source for such educational video lectures prepared by professors of esteemed IITs and you can benefit from such knowledge.

Again, what you do in the gap year solely depends upon your area of interest.

Good Luck