Lots of different models of this. All are operated by a single SG90 type servo and have been adapted for different scenarios. Examples are the rubbish collecting robot and the hexapod insect robot.
The servo is either an SG90 or MG90 type. One arm is retained with a short M3 nut and bolt, the other attaches with a servo horn. The green block has two recessed holes for attaching the gripper to a mount as required using M3 bolts. The gripper arms have a hole at the end of each, this is to facilitate adding an attachment such as a paddle for greater gripping area, or to add an object detector to prevent over tightening the grip. I've done this with just two wires and using objects made of conductive foam.
The .stl files for these are downloadable below but the original design can be accessed on Tinkercad by searching for Tekyinblack/SG90 Gripper 1