First go to Settings->General and enable Google Input Tools:
Scroll all the way to the bottom and save the changes:
Now you can configure the languages you want to use in gmail, after which you will see an icon with अ in the top right corner of gmail window:
You can toggle this by clicking अ
And now when you type in english, it will transliterate to Samskritam!
Spread the word and tell others about it.