are you marking stitching lines or quilting lines?
if we're talking stitching/seam allowance, and your pieces are small enough to fit, try using computer generated templates and printing on the back of your fabric. if you have an HP printer and don't pretreat the fabric, every bit of the ink comes out the first time you wash it.
if you mean marking the lines for quilting it after the blocks/top are pieced, i've only found 2 pens that are worth the space they take up in my drawer. those are the dritz (or no-name) purple and blue pens. purple fades away on its own; blue washes out.
Those gel pens sound interesting. LP"Mom" ... do the lines wash out? Does the white show up well on black fabric? if yes to both, where do you get them?