Wipe the tip of the gel pen off with a paper towel or clean cloth. The ink can clog the tip; cleaning it off can help it to clear and run free again.
Keep the cap on the pen at all times when the pen is not in use. Air can cause the gel ink to dry in the tip, which will cause the ink to become stuck.
Scribble in circles on a piece of paper. This will help the roller-ball tip to move and rotate, which can draw the ink to the tip and allow it to write. This method may take a few tries, but it is generally the most effective way to get the gel pen to write.
Run the tip of the pen under warm water. The water can loosen the ink in the tip and help it to become unstuck.
Store your gel pens upright in a pen holder or cup rather than allowing them to lay flat. This can prevent the ink from gathering and becoming stuck at the tip.