All your Gmail Email Templates are stored in your Google Drive folder called Email Templates:

So if you want to restore a previous version of an email template you can use Google Drive’s revert to earlier file versions feature.

Here is how to do that:

1. Right-click on the template you want to revert to a previous version and select Manage versions. You can also click on the version icon in the right panel:
   
2. Download the previous version of a template:
   
3. To revert to an earlier version, click Upload New Version, find the file you have downloaded in the previous step, and click Open.