I've been looking at this diagram to try and understand what's what under my hood, and I'm not sure what "GB-1" and "GB-2" and so on are referring to.
It is a ground, and as Digital Lightcraft "Ground Bolt" sounds reasonable. The picture below show another view from the same car. You can see from the picture that it is indeed a terminal that's bolted to the chassis.
Hard to say without knowing the car, but based on previous experience, its likely to be "Ground Bolt"
IE ground / earth (-) for the electrical system to the chassis/shell/monocoque