GTKeyboard comes with several different foreign keyboard, at this point in time including Spanish, Italian, German, American, UK, and French keyboard layouts. If you don't see support for the national keyboard you prefer to use with GTKeyboard, see the KEY/ directory of the source distribution, which contains notes on how to create a keyboard for your country. If you create any keyboards that GTKeyboard doesn't have, I would greatly appreciate hearing about it so I can include it in future releases of GTKeyboard.
To change keyboard layouts, just click on the Options menu, and select the ``choose layout'' menu item. It will bring up a file selection dialog which you can use to select the .key file corresponding to the keyboard that you want to use. Multiple key files are installed wherever you installed GTKeyboard, which will most likely be in /usr/local/share/gtkeyboard/ - but it may also be in /usr/share/gtkeyboard/ depending on how your copy was installed. GTKeyboard will try to choose the correct directory where the files are located by default. After you have chosen the correct file, press OK in the dialog, and the keyboard will remap itself. At this point, if you wish to use the foreign keys in a redirect window, you will have to remap the keyboard before continuing. Otherwise, if you just want to edit text, then you don't have to do anything extra.