Gmail TD Button - shared user cant cant open email
The email can't be read by people with who the task was shared. I would suggest that the content of the email is taken and posted in the task so that when other persons view the task they can read the email as well. This could be an option to have the test copied and attached to the task.
Also I would not make it a link for persons who does not have access to the email, as at the moment it opens gmail, it just shows the inbox of the shared person.