> I am looking for a database modeling tool

did you have a look at www.ibexpert.com
Database Modeling can be found in IBExpert Menu Tool-Database Designer

>that is either GPL

no, it is not GPL

>or inexpensive.

199 Euro


to get a first impression what it does, just download the trial
and try it or have a look at
http://www.ibexpert.info/documentation/17088.html

Best Regards

Holger Klemt

The Worldwide Firebird Conference
November 13th - 15th, Prague, Czech Republic

The most Expert for InterBase and Firebird --- www.ibexpert.com
HK Software - Gerhard Stalling Str 47a - 26135 Oldenburg - Germany
Phone/Fax +49 700 IBEXPERT (42397378) www.h-k.de
Training and Support for Delphi, InterBase, Firebird, AS/400