GUC - Grand Unified Configuration
Grand Unified Configuration") is PostgreSQL's configuration system.
The Grand Unified Configuration (GUC) system was introduced in PostgreSQL 7.1 as a way to control PostgreSQL at various levels.
The overview in src/backend/utils/misc/README provides an introduction to adding configuration parameters, and is relevant for both for the core code and extensions.
See also src/backend/utils/misc/guc.c.