Steps to enable Tamil Keyboard layouts in GNOME Desktop:
1. Move the mouse pointer over the (top/bottom) panel
2. Right click.
3. Select the “Add to Panel..” option
4. Select the “Keyboard Indicator” and Press ADD and close.
Now you will see the string “USA” in the panel.
5. Now right click on that
6. Select the “Keyboard Preferences”
7. Goto Layout Tab and Press Add
8. Select the country “India” and Variants “Tamil Unicode” or any of other tamil layouts and Press Add.
Now, the newly added layout will be listed.
9. By default, you can change the keyboard layout, by pressing both ALT keys togather. You can change that by going into “Layout Options..” -> Layout Switching.
10. You can also change the layout by left clicking on the Keyboard indicator applet.