Here is the link provided to how to use the IME tools:
You will find all the help in the above help link provided by Gmail Team itself directly.
Hope this will be most useful.
Earlier also the option of सं was there, but it could be enabled or disabled from the inbox itself, by clicking on the icon. Now it appears in the right side of the Inbox only when enabled and you have to disable or enable using the drop down menu in the IME icon, by the side of the gear icon. I think now Vishvas has made some changes such as using it in Gtalk also and some more changes.
I am using Baraha IME which is always available in all the Windows Editors, and Inbox or any web page and easily enabled or disabled by clicking on F11 button. Only the handicap is it is not usable in Window7 or later versions of Windows and the other OS than Windows.
The IME modified Google Transliteration is available on all platforms irrespective of OS.
I had used sometimes, the earlier version of Google Transliterate and made use of it. Only you can select the right characters when a key can have different combinations, by using backspace. Now I think this facility is disabled as it is updated with keyboard input method by Vishvas.
With regards