Traditional gouache is usually sold in tubes and has the consistency of toothpaste. I suggest thinning with water to the consistency of whole milk. Other additives make it perform differently. Gum arabic is a popular binder in tube gouache (and watercolors), and adding more of it makes the paint "puddle" better -- reduces flow and bleeding. Ox gall has the opposite effect of increasing the flow, which also makes the paint run and bleed more readliy. Play with these additives, 1/2 drop at a time in a few tablespoons of paint, until you are pleased with the flow through your pen, which is largely a matter of personal preference.
Since traditional gouache is basically pigment and binder, it remains water soluble after drying. That means if you put a watercolor wash between two lines of it, the lines will probably bleed.
Acrylic gouache, aka airbrush color, handles similarly, but it is generally sold in liquid form. It is too thick to run through a pen without thinning first with water, but I would advise against putting anything else in it. A very small amout of added water makes it run much better. Again, experiment until you are happy with the flow characteristics.
Acrylic gouache is water soluble when wet, but not when dry. So, when you put a watercolor wash betwen two penned lines, it will not bleed.
Wally, if acrylic gouache is properly thinned, you should be able to run a large-reservoir ruling pen dry without clogging. That's several feet of ink line on a mat. Do you thinning in a smaller, separate container, not the whole bottle. If the color is no longer opaque, then you've added too much water.