Hello! I'm finishing up my undergrad and ended up applying this semester. Unfortunately, my timeline was not planned out so well, so I ended up taking my general GREs in July and squeezed in my subject test in September. I wrote my personal statement over winter break in addition to working one of my jobs and doing another project - it was incredibly stressful. Despite the time crunch, I don't regret applying this year, and I was lucky enough to get into two of the programs I applied to. I would not have been able to take a gap year for a variety of reasons, but financial reasons were the most pressing. If I hadn't been accepted to programs this year, my plan was to switch gears and find a corporate job, with the possibility of revisiting the idea of grad school later on down the road.
Throughout my time as an undergrad, I have worked a variety of jobs, one of which was in the industry my English major has prepared me for. While the job was great experience, I learned pretty soon into it that the industry was not for me. That being said, over my three years there, I was able to pick up a lot about how the corporate world works, along with other essential skills for surviving in business. As a side note, I was home-schooled throughout high school, so I haven't really felt the "burn out" that I've heard many students talk about - but I could imagine that being in a traditional school for almost two decades would make one feel the need to take a break before jumping into another 6-8 years of study.
Overall, I definitely see the benefits of a gap year. I'm very happy that I applied during senior year, despite how incredibly stressful it has been. However, taking a gap year would probably help mitigate some of the stress you'll already be feeling during senior year. I would strongly suggest considering the stress and anxiety that's caused by applying while you're in undergrad, because it can take a major toll on your health.
I know that my experience is anecdotal, but I hope that hearing a variety of experiences helps you make the decision that's best for you.