One of the main ideas of GF is the use of libraries in grammar writing, in a way familiar from software engineering. In this way, large grammars can be built in cooperation, and old grammars or parts of them can be reused in new grammars. The slides Grammars as Software Libraries give some introduction to this idea.
This library covers basic linguistic structures of different languages.
Version 1.0 (still beta, but already recommended). Covers Danish, English, Finnish, French, German, Italian, Norwegian, Russian, Spanish, and Swedish.
Version 0.6. Old version with 7 languages.
The prelude library gives utility functions for different GF applications:
Precedence. Utilities for formal languages: precedence levels, associatives, infixes.
Predef. Type signatures of predefined (hard-coded) functions.
Prelude. Generic utilities for strings, tables, records, booleans.