Hello! I have been having issues with my Canon Pixma G6020 printing with these horizontal lines. This is a new issue, these lines have never appeared before.
I am using the exact same paper and exact same print and quality settings I have always used. I have tried almost everything under the maintenance menu besides the ink flush: I have done cleaning, deep cleaning, head alignment, roller cleaning, btm plate cleaning, all multiple times with no luck.
In this situation, you can try reseating the print heads using the steps in the link HERE to see if it resolves your issue. If there is air in the ink lines, it can cause gaps or banding in the prints. If reseating the print heads and cleaning have not resolved your issue, you can try the ink flush to see if it improves the print quality.