Drill a small hole in the gripper so a paperclip can fit through it. Once the hole is drilled and the gripper is slid onto the top of the arm, measure the length between the hole and the servo horn. This will be the length of the paperclip section needed to allow the gripper to close and open when attached to the servo horn.
Bend a section of the paperclip with needle nose pliers so it looks like this:
It may take a few times to get the paperclips the right side and bent straight enough to allow a smooth movement on the arm rails.
If there is resistance between the gripper and the arm rail when sliding, smooth out the edges of the arm rail with a blade and/or sandpaper.