There are also many kinds of watercolor pens, which can also be used to draw gouache, but there are still differences. Usually, oil painting pen is the hardest, gouache is the second, and watercolor pen is the softest.
If you can't buy a professional watercolor pen, buy a sheeptail flat pen in gouache, which is very common; The watercolor pen can be replaced with the wolf hair in the national brush. The effect is enough.