Eight weeks after the official rollout announcement in July, all users will be automatically updated to the new Gmail, though they'll be able to opt out for another four weeks.
The third option would be the default setting as well. 8 weeks after that, anyone who hasn't done that will be automatically transitioned to the new Gmail service, with a 4 week period to opt out (but only during that period.). "They'll have the option to opt out of the new Gmail for an additional four weeks". However, the new UI will roll out to all G Suite users in July, and we'd expect the same to be true of regular Gmail users.
New Gmail is now available in an "Early Adopter Program" (EAP) with an option to try it out before the GA. If you prefer the old design, or just aren't ready to make the change quite yet, you can open the Settings menu and click "Go back to classic Gmail".
For more, be sure to read 5 New Gmail Features to Check Out Now.
This can house Google Calendar, Google Keep and Google Tasks all in an easily-accessed bar.
Google plans to provide further transitions in details in July but is encouraging admins to start testing it with users now. Then, if you still hate it, you are out of luck because come October, you will be using the new Gmail no matter what.
Gmail spam mystery: Why have secure accounts started spamming themselves? . Here's how to use the Gmail improvements, which are ready now.