Some tips from Sakura, maker of the Gelly Roll pen, will make a gel pen that still has ink in it to work again.
Make circular motions on paper with your gel pen. "The motion makes the tip (ball) roll and causes the gel ink to be pulled into the ball tip chamber. It might take a few tries," according to the manufacturer's FAQs.
Do not tap the bottom or top of pen; this may cause the ink to jam.
Heat the point with a lighter for a few seconds to warm the ink and loosen it. Do not touch the flame to the pen.
Hold the gel tip pen over a kettle or pot of boiling water for one to two minutes to heat and loosen the ink.
Leave the cap on your gel pen at all times when the pen is not in use to prevent it from drying out. None of these tips are guaranteed to work. Some pens may be too old or too dried out. Be careful if using a lighter on your pen.