I did a gap year before grad school as well. A friend of mine and I got a grant to travel around the country to photograph and do research on the history of firehouses. Columbia was happy to hold my admission a year. My son’s girl friend worked for an NGO in New York for a year while she applied to PhD programs. She just felt she couldn’t really enjoy senior year applying to programs and she wasn’t 100% sure she wanted to be an academic. She got into excellent programs in her field and learned quite a bit about what it’s like to live in NYC working for an NGO.
Many eons ago I took a gap year. I’d taken Calculus as a senior and got a 2 on the AP exam. Still don’t know how that happened! So I retook Calc as a junior in college (so after three years of no math). It was so easy this time. I took it self paced, finished the final before the half-way mark and ended up being asked if I’d like to correct homework the next year. Senior year I took Physics 1. I’d had biology as a sophomore, and chemistry as a junior in high school. It was hard, but I hovered between an A- and B+ and ended up with a B+.