The database schema for GL2 is still in flux. The last thorough schema draft, created by Dwight Trumbower, includes partial schemas for several modules. You can see a visual representation of the database here and MySQL DDL here.
A more current, though less pretty, version of the schema can be found in the Geeklog 2 CVS. Note that the schema is still likely to go through several more permutations before reaching a final state.