Use the gel pen to make circles on the scratch paper. Often the circular motion causes the tip of the gel pen to roll and when this happens, fresh gel ink will flow into the chamber of the tip. Continue drawing circles on your scratch paper to make this happen, pressing firmly to make the fresh ink flow.
Draw a fine, long line onto the sandpaper. Often the grit of the sandpaper is enough to make the gel pens release ink. If the sandpaper does not work, try drawing a fine, long line onto a paper towel. This creates the same effect where the gel pen may release ink into the chamber.
Make a line on your skin. Often the oils in one's skin will be instrumental in unclogging gel pens.