MarthaT, Pilot makes three types of Frixion gel pens...a fine point, a ball/roller point, and a highlighter. All are erasable on paper, but there is no mention of use on fabric made by the company. They are intended for use on paper and that's how they are marketed by Pilot. On paper, the thermo-sensitive gel markings "erase" from the friction of rubbing with an eraser. Quilters have 'adopted' them, just as they adopted Orvus (a shampoo for horses) and many other products.
I can't/won't tell you what to buy for your quilts, but personally I use a Sewline ceramic pencil with either white or gray lead depending on the fabric I'm marking.