Gel Pens A gel pen uses ink in which pigment is suspended in a water-based gel. Because the ink is thick and opaque, it shows up more clearly on dark or slick surfaces than the typical inks used in ballpoint or felt tip pens. Gel pens can be used for many types of writing and illustration. What is the difference between gel pens and ballpoint? The main difference is the ink inside that reservoir. The ballpoint uses a thicker, more viscous ink, rather than straight liquid. ... Or they can use a gel ink. A liquid ink rollerball basically puts more ink onto the paper than a ballpoint.