That is a great idea. They could make short work of the situation or at least shorten the height of a bunch of Muslims. Now what about the Sikhs?
The silence and darkness of night, methodical selection of targets, and their up close attack with hand to hand tactics is exactly what is needed to sever the command and communication structures of ISIS/L.
Involving Indian Sihks would bring more troubles to India, which is already unstable with Islamic unrest courtesy of Pakistan and their agents. The Gherkas are seen as extensions of the British Crown more so than of their Indian heritage.