Ferret is the GNU data modeller.
Ferret (formerly known as GerWin) is the Free Entity Relationship and Reverse Engineering Tool.
It lets you create data models and implement them in a relational database. You can draw your data model via an entity-relation diagram, generate the tables from it (another graphical diagram), and then generate the SQL that creates such relational tables.
Several SQL dialects are supported (for most popular free software database systems) and it is very easy to patch the sources to support more.
How do i run Gerwin?
should work if you have tcl/tk installed in your GNU system.