Gel pen uses ink, pigments suspended in a water-based gel. Because the ink is thick and opaque, it is more clearly visible on the surfaces than the typical inks used in pen or felt-tip pens. The insert in the holder, can be use for many types of writing and illustration.

The overall design of a gel pen is similar to that of a conventional ink-based pen, with a barrel containing the writing mechanism and a cap, and a tank is filled with ink. The barrels can be created in a variety of sizes and designs; with a finger grips are made of rubber or plastic. The tip size is in the range from 0.18 millimetres (0.0071 in) to 1.5 mm (0.059 in.).