Here are the details of the response I received from Google for this issue
Thank you for contacting Google for Work Support. I understand that
  that you Gmail Contextual Gadgets have disappeared again from the
  Gmail web interface. 
I notice there are other user reporting a similar behavior hence he
  have requested an update from our Eng team and I am currently waiting
  for their response. As soon as I have an official response, I will be
  contacting you back directly with the next steps. 
Also I notice you mentioned that the previous workaround is no longer
  working. Previously the workaround provided was to press "Shift" and
  then click on the email. The email will be opened in a new window and
  then the gadgets should be displayed. Can you please confirm if the
  above workaround is working or not?
I’ll keep this case open while I wait for your reply. If you have any
  other questions or additional comments, don’t hesitate to reply and
  I'll be happy to continue helping you.
Sincerely,
Wilmer Google for Work Support
Here is the latest update from Google
As before this issue appears to be UI based as opposed to any problem with the Contextual Gadget API itself, we've gathered some troubleshooting information from affected test environments and are working on identifying a clear root cause and then remedying the behaviour at the moment.
There isn't any further you need to do on your end, I'll update you as I learn of any changes in the status of this behaviour.
Regards,
Patrick