Have you ever tried to write with a gel pen on paper that you had coloured with pencil crayons? If so, you probably noticed that gel pens and pencil crayons don't get along. However, there is a solution:
But this marker is not completely useless. Use it to colour over paper that has been coloured with pencil crayons, and you will create a surface that is friendly to gel pens. The marker does not change the colours of the pencil crayons, but it does blend and soften their colours.
Here is my test, using a Staples Gel Mini, which is an acceptable but not great gel pen that barely writes at all on top of pencil crayons:
You can see that the gel pen wrote much better in the second example, where I used the white paint marker over top of the pencil crayons. You can also see a bit of that softening and blending that I mentioned earlier.
You can also avoid some of the pencil crayon-gel pen problems altogether by using a better quality pen, such as a Sakura Gelly Roll, which writes on almost everything. But even then the marker still helps the pen to write more smoothly and the colours to show up better.