Simply put, they're purple cosmic annoyances with paddle like feet that harass any civilization they come into contact with, and they gladly pick up the weapons generated for them in their natural habitat. A more in depth explanation, however, is below;
Giboblins are a naturally brutal species, taking major advantage of the weapons that the western Tube of Terror provides on a random basis to attack unlucky civilizations in and around the Tube. All it takes for a plague of them to start is a single Giboblin spitting in the right place, furthering their danger. Thankfully, they have little in the way of intelligence aside from using weapons and are incapable of civilization building.
The western Tube of Terror serves as their only natural habitat, generating weapons for them. Giboblins exported to other places grew docile and became easy to eliminate, even if weapons were introduced a few generations in.