These are called Frixon Erasable Gel Pens by Pilot and the package says they are removable with friction (ie, by erasing). However I found them on Patsy Thompson's web site and she says they are removable with heat. Another site says the the ink is thermosensative and becomes invisible when ironed at 140 degrees or higher and the marks will not reappear unless the temperature drops to under 14 degrees F. This makes them OK for all my quilts that stay in Hawaii.
Anyway, feeling better and looking for something to keep my hands busy so they wouldn't be tempted to rub my still red and itchy eyes, I tore open the package, grabbed some fabric, made all kinds of marks and then turned on the iron. The marks were completely gone after one swipe with the iron.
This video shows the pens in action. It's from E E Schenck, a quilt supply wholesaler:
The set I got includes eight pens in black, blue, red, green, purple, pink, orange and brown.