Basics:

With Gmail you can use any addressbook program which supports drag'n'drop. The best choice is the 'GnomeCard' address book which comes in the gnome-pim package.

You can drag'n'drop entries from GnomeCard into the gmail compose window. You must drag it into the to, cc, or bcc fields.

There is a menu item and a tool bar icon in the compose window to run your address book program. The default is GnomeCard, but this can be configured in the preferences.

Unfortunately Gnomecard does not support drag receive, so we cannot easily add new addresses from Gmail to Gnomecard. An addressbook program does not belong as part of gmail. I will make sure Gmail will work with any addressbook program that supports drag'n'drop.