(Andrew correct me if I'm wrong)
1. You can use AppGen's 4GL language
2. You can access the database using ODBC drivers
3. There is a C interface
and there might be other options.

Spending about ten minutes googling I didn't find anything that seemed to fit your needs using Java. I'd avoid VBA.

I suspect using AppGen for the GL linked via database interface (ODBC) to your custom inventory code would work fine.

Tony