I have read extensively on creating Gmail contextual gadgets but am unable to find information on creating a plugin/addon that users can install on their Gmail accounts to insert specialized text.
The idea is that a user would install the gadget / add-on and in certain emails they would click this button to enable insertion of tracking codes into the body of the email.
Any suggestions would be much appreciated.
Thanks in advance
The gadget does not have access to the Email for modification...
I think are you wanting to modify the DOM of the email.
Your best bet to do this would be to use a browser extension. InboxSDK is one that I have used. This SDK in Chrome for example will do just what you want.