A gripper I have made as part of my college graduation project, based on the design of jjshortcut, included on his Robotic arm with 7 servos (see link on the left).
In my version, the mechanism is moved by a linear actuator (used to adjust automotive headlights) instead of by a servo motor. Also, unlike the original, I've used a robotic construction kit to prototype the main structure of the arm, so my gripper have two holes on each side that will only fit parts included in the kit. (see image on the left).
Lastly, if you do need to customize but don't know the first thing about how to create round toothed gears from scratch, you might also be interested in reading a document I wrote, compiling my experience on the subject so far: http://www.stefanomega.com.br/thingiverse/geartut/index.html
First you'll need to create the custom parts by using either the 3D STL files (CNC milling, 3D printing) or the 2D DXF, EPS and PDF files (CNC milling, laser cutting), plus bolts, washers and nuts (see below).
I have prepared a simple PDF containing all required assembly instructions, including part names and the bolts, washers and nuts specifications. You can download it at: http://www.stefanomega.com.br/thingiverse/stefano_manual.pdf