I used KDE System Settings to configure my KDE appearance settings (I wanted to make KolourPaint look native), even though I use GNOME. I changed some font settings through it, and now all of my fonts are really choppy. Is there any way to revert these changes?
All font settings are stored in a hidden file in your home folder, called .fonts.conf.
If you remove it (in your home folder, press Ctrl + H, and delete .fonts.conf), and generate a new one, by running, in a terminal instance (Applications > Accessories > Terminal):
sudo dpkg-reconfigure fontconfig
You then should get the usual Linux crystal clear fonts (you actually may need to log out and log back in, but that's it)