We need to clear up a misconception first: ‘gel’ refers to the type of ink used in a pen and not specifically to the style of tip, although gel ink is commonly used in rollerball pens.
The tip technology in a ballpoint and a rollerball consists of a metal cone supporting a metal ball, which transfers ink to the page. In a rollerball pen the ball tip tends to be more spherical, delivering a smoother writing feel to that of a ballpoint pen, which, depending on the quality of the ball, can feel a little scratchy.
Gel ink uses a pigment suspended in a water-based gel. This makes the ink thick and opaque and results in dense, rich colour that shows up well on most paper, especially dark coloured media. Gel ink is often used to carry special additives, such as metallic flakes that give the ink a sparkly effect.
In addition to its vivid colour and smooth, non-skipping quality, gel ink is acid-free, waterproof and fade-resistant. Some pens are archive-quality.
Rollerball pens are also available with dye-based (water-based) ink. This is slightly thinner than gel ink, but still extremely smooth to write with. The metal ball tip picks up the ink and rolls it on to paper to give smooth, even coverage.
When rollerballs first became popular in the 1970s they caused a sensation, because the ink flowed smoothly, like water-based ink in a fountain pen, but without the scratchy feel of some of those nibs.
Both gel pens and dye or water-based ink rollerballs require very little pressure to write and, because the ink is not fed by gravity, they write at any angle.
Ballpoint pens use oil-based ink, which is thick, like a paste. Because the tip lays down less ink than either a gel pen or a water-based rollerball the ink in a ballpoint pen can last much longer. However, ink coverage over the ball tip is not always consistent, which can result in skipping or blobbing, particularly towards the end of a refill. More pressure is required to press the tip to the surface and deliver the ink to the paper. As a result, some users find writing with a ballpoint over long periods less comfortable than using a gel or waterbased rollerball.