There are a few alternatives, since gel pens can be iffy—chalk markers are an up-and-coming tool a lot of people use now. They used to be used for like cafe signage and stuff, but now chalk markers can be used for almost everything art-like……plus there are various size tips now. In a similar vein, paint pens also work—there’s Posca and similar pens. Chalk markers are probably the easiest and least expensive to find—like a paint pen, you shake it and press the tip to prime it…….but I’ve found they work pretty good on all sorts of marker works.