As a reseller, the default WHM that comes with the your account will not send welcome emails to your clients. A separate billing system would be required in order to have welcome emails sent. Hostgator provides WHMCS or WHM AutoPilot billing systems for you to install.
WHMCompleteSolution (WHMCS):
To edit or create your email templates, go to Setup > Email Templates inside of your WHMCS admin panel. If you would like to create a new template for an email that you may send regularly but manually, scroll to the bottom of the Email Templates page and click on Create New Email Template. From here you can add your new email template.
WHMCS provides more information on this topic in their Support Wiki
WHM AutoPilot (WHMAP):
To edit or create your Email Templates, click on the Utilities Tab > Email Templates inside of your WHMAP admin panel.
You should now see Admin System Templates and Order System Templates, the difference being that the Admin emails will be sent to you on their respective actions while the Order emails will be sent out to your clients. Pick the category you would like and click Templates.
It will now list the different Email Templates that are available to you. Just click on Manage and proceed to format the Email Template in the manner you would like. Different variables that can be used in the email should be listed at the top.
WHMAP has a video tutorial on this topic on their Support Site, just click on Email Templates under the Utility Tab section on the right.