Warm up the gel pen by rubbing it between the palms of your hands for a few minutes or holding it under hot, running water. The heat will get the ink flowing again.
Attempt to get the ballpoint tip of the pen rolling again by drawing small circles on a piece of plain, white, copy paper. Do not press too hard, or the roller ball might get jammed inside.
Replace the cap on the gel pen to prevent it from drying out in the future.
Store the gel pen in a cup or container with the tip pointing down to keep the ink near the ballpoint.