The new Gmail web interface has been available for over a month now, but it's still in preview and it's an opt-in affair. 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.
When the new interface launches in July, G Suite administrators will have three options: immediately transition their users to the new UI (with an option to opt out), allow their users to opt into the new UI, or wait about four weeks until their users are allowed to opt into the new UI. The last option will be the default setting.
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. But 12 weeks after the new UI is officially announced, all users will be migrated to it with no option to opt out. It's a good thing that the new UI isn't too controversial.
If you'd like to check the new UI out now, you can opt into the Early Adopter Program simply by hitting the settings button in your inbox and selecting "Try the new Gmail."
- G Suite Updates Blog