Fountain pen or ink roller inks:
The dyes are entirely dissolved in a liquid.
The ink has a very low viscosity and penetrates paper quickly and easily.
This may result in the ink smudging or permeating the paper.
Gels:
Unlike ink, most gel types contain pigments. (These are small coloured "granules".)
Due to their coarseness, pigments do not penetrate paper so easily and thus remain predominantly on the paper surface. Smudging and/or permeating of the gel is thus generally not an issue.
Furthermore, pigments have the advantage of not colour-fading as readily as dyes when exposed to UV light – for the most part, they are more light-resistant.
Moreover, gels can also be lustrous, fluorescent or indelible.
Due to their higher viscosity gels have a stronger propensity for drop formation (blots or blobs) than inks.
The metallic ingredients of the metallic colour gels tend to sediment, i.e. they accumulate after extended "tip down" storage, resulting in a non-homogenous colour release.
