Strike-slip faults are a type of transform plate boundary. This means that the plates are sliding past each other horizontally, with little to no vertical movement. The earthquake in Gansu was a result of stress building up along this fault, which eventually led to a sudden release of energy in the form of an earthquake.

It's important to note that while the specific fault responsible for the Gansu earthquake is not explicitly mentioned, it's likely related to the Altyn Tagh fault or the Kunlun fault, both major strike-slip faults in the region.

Therefore, the plate boundary involved in the Gansu earthquake is a transform plate boundary, specifically a strike-slip fault.