These are executables or shared libraries doing the conversion work. They map the CORBA calls to the datasource specific calls. They may use ODBC, a native library for the database, XML parsers and generators, LDAP, POP3, whatever you can imagine.
A server framework is provided with GDA so that it's quite easy to add a new server. This is done by using a unique command which lets you create the basic structure of your server and build it.