You can still open Gmail: just click the new "Gmail" menu item, which is placed below "Contacts" in Inbox's sidebar. The URL that temporarily disables redirects is https://mail.google.com/mail/?ibxr=0.
"If you often go to Gmail in your web browser when you really mean to go to Inbox, you can turn on a setting so that you'll automatically be taken to Inbox when you visit gmail.com or mail.google.com on a computer. You can always get back to Gmail using a simple link in Inbox," informs Google. The setting was added a few weeks ago.