What is the official name of this image ? It is often found on the side of a panel widget for dragging.
Is it named "gripper"? A Google search doesn't return a lot of results for "gripper icon".
Very few UI elements have names that approach anything close to being 'official'. There's plenty of common terms for a lot of the common UI elements, but even then, one shop may call it one thing, while another a something different.
So, use what you want and your team agrees upon. "gripper" is as good as any.
The jQuery library calls this icon 'ui-icon-grip-dotted-horizontal.'