11th October 2018: PostgreSQL 11 RC 1 Released!
Supported Versions: Current (10) / 9.6 / 9.5 / 9.4 / 9.3
Development Versions: 11 / devel
Unsupported versions: 9.2 / 9.1 / 9.0 / 8.4 / 8.3 / 8.2

52.3. Implementation

Internally, a GIN index contains a B-tree index constructed over keys, where each key is an element of the indexed value (a member of an array, for example) and where each tuple in a leaf page is either a pointer to a B-tree over heap pointers (PT, posting tree), or a list of heap pointers (PL, posting list) if the list is small enough.