To revive a dried-out white gel pen, there are two tried and tested methods: shake it vigorously or blow air through the ventilation hole. If an air bubble obstructs the ink flow, shaking vigorously and blowing air will help remove it. If shaking does not work, it is advised to soak the cartridge in warm water to liquefy the ink.
If you are unable to restore the gel pens to functioning condition despite these alterations, it is recommended that you trash the pen and purchase a new one.