Will taking a gap year after my Master's degree improve or degrade my potential of doing a PhD?
If you're using the gap year to get real-life work experience, it will improve your potential to do a PhD. The reason is simply that you'll have a clearer idea of why you're going to grad school.

I've seen many students who weren't ready to be adults even at the end of their undergraduate degree. They were heavily sheltered by their parents, and never had to worry about things like money. When money isn't an issue, university becomes something to be attempted because it's there. The post-undergraduate job search becomes heavily intimidating, and going to grad school the easy way out. So they end up in grad school with no real idea why they're there.

Real-life work experience goes a very long way to broadening one's world view. You'll learn what's needed to succeed, what's needed to be able to afford a house/car/spouse/children. You'll learn that you only have so much time to spare and that grad school might not be worth it. And if you do choose to go to grad school after all, you'll have a much clearer idea what exactly you want out of it. Since motivation is a key factor in any endeavor, this improves your ability to complete the PhD.

Only drawback is, it's my impression that grad schools like to admit starry-eyed students who're doing PhDs because they want to become professors. Having different career goals isn't fatal to your application, but you might have to modify your statement of purpose somewhat.