For regulations surrounding guest difficulties, see: Ranking Criteria
A guest difficulty (or GD for short) is a beatmap of a beatmapset that is not created by the beatmapset host. They can usually be identified by their difficulty name containing the guest creator's username. A beatmap featuring multiple creators is also known as a collaborative difficulty.
Many guest difficulties are often requested by private messages between creators. Although GDs can also be requested in modding queues.