Basically, here is how things are organized:
Mergeant uses libgda as a unique mean of accessing a database (at no time has Mergeant knowledge of what the actual DBMS is), and gnome-db for some of its GUI.
Libgda is composed of a framework, and of several 'providers' which each enable the access to one DBMS. These providers are in different state of usability (consult the gnome-db/libgda site for more informations). There exist a provider for all the most known DBMS (Oracle, Sybase, PostgreSQL, MySQL, etc). To have Mergeant working correctly, a version of libgda and gnome-db which is correctly installed and configured is required.
Also, using Bonobo the user will be able to do some tasks specific to the DBMS (managing users and their rights, managing databases, etc), provided that libgda supports it for the DBMS (this is broken at the moment).
Here is a diagram to summarize it all: