Re: gel pen
Without knowing the formulation of the gel, there's no way to tell except by experimentation.
A paint marker might take (that is, adhere to the surface enough to stay put), once the surface of the painting has cured, but paint markers are not well-known for their elegant or thin lines, or for their elegant color.
With practice, a script liner or rigger brush can make deliciously fine lines using thinned oil paint. I think I've seen these down to 10/0, or about 3 hairs. A size 1 rigger / liner makes quite a thin line.
If you need straight lines, a bridge or mahl stick or (a la Richard Schmid) cheap hospital cane can be useful. Freehand curvilinear lines require practice.
Judging a Manet from the point of view of Bouguereau, the Manet has not been finished. Judging a Bouguereau from the point of view of Manet, the Bouguereau has not been begun.
--Robert Henri, The Art Spirit