Use this to get the revert to the old look option back on the gear.
qwerjk.com/revert-gmail
It's temporary but works.
If you want the HTML interface, click the HTML link when you reload the current look (but you have to be quick) otherwise this link will get you there
https://mail.google.com/mail/h/sdalnvllj81o/?zy=h&f=1
You can set HTML as the default with the links at the top of the HTML view.