If the act of moral turpitude is not connected with the company (for example, rape in his personal life), then why do you want to hold the gratuity of the employee? Tell the employee to submit their resignation immediately, and that is all. Loss of employment, in itself, is punishment.
Of course, this is my personal opinion. In personal life, for whatever has happened, the guilty has been punished under the IPC. Now, if the company also punishes, then it will be double punishment. For any offense/conviction, there can be only one punishment and not two. Therefore, please think twice before you invoke this rule. Please check the standing orders as well.
I say this because the loss of pecuniary benefits will cause hardship to the guilty's family members. In a real sense, they will be punished more than the guilty itself.
Thanks,
Dinesh V Divekar