The PostgreSQL source distribution includes several examples of index methods implemented using GiST. The core system currently provides text search support (indexing for tsvector and tsquery) as well as R-Tree equivalent functionality for some of the built-in geometric data types (see src/backend/access/gist/gistproc.c). The following contrib modules also contain GiST operator classes:
btree_gistB-tree equivalent functionality for several data types
cubeIndexing for multidimensional cubes
hstoreModule for storing (key, value) pairs
intarrayRD-Tree for one-dimensional array of int4 values
ltreeIndexing for tree-like structures
pg_trgmText similarity using trigram matching
segIndexing for “float ranges”
Read the upstream documentation. Report corrections through the PostgreSQL documentation form.