I get a lot of questions about white gel pens and the problems people have with them. They are a problematic tool, but I've done pretty well with them, so I'll pass a few of my observations.
The ink collects on the nib and dries, impeding the ink flow. Every few minutes, wipe off the tip. If you see or feel hardened ink chip it off (I just use a fingernail).
Draw slowly. The ink is thick and doesn't flow quickly. You'll get a more even line if you draw sloooowly.
The pens don't last very long, but the ink will dry on the sides of the pen too, so if you've used one quite a bit, it may be out of ink even though it doesn't look like it.
Sometimes you may have to go over an area more than once to get the white you want. Despite the fact that the ink dries so easily on the pen itself, it takes a while to dry completely on the page. If your ink just seems to smear or get dirty when you are trying to add more white then it hasn't dried, and you are just pushing it around. Wait longer and come back (I often wait up to 1/2 hour--it depends on your paper).
To get a graduated look, I draw a few lines with the gel pen, then smear with a finger, immediately. The white disappears, but it lightens the area. Then I add more white exactly where I want the highlight. (I haven't had luck using a paper stump--it just picks up the ink).
To get an 'embossed' sort of look draw your tangle with black pen, then use the gel pen to draw a line right next to the black line. Sometimes I only do this on one side and either the top or bottom. Sometimes I do it all around--you get a slightly different look.
It seems obvious, but I'll say it anyway--the darker the color beneath, the better the white gel looks.
All said, Gel pens skip no matter what you do, and sometimes you just have to be patient. Some people swear by one kind over the other, but I've had mixed experiences with all of them. I think the type of paper you are using makes the most difference. They aren't too expensive, and I do love the effect they give, so I put up with skips.