Google has released an updated UI for Gmail in February. The new look brings Meet, Chat and Spaces and applies more of Material You style effects.
From now on, the new look will be released for everyone. So your account will change to the new UI very soon.
It's not a huge change, but Google seems to be trying to make its Workspace suite look popular. This of course does this to make Office more competitive.
According to Google, you can choose which applications will be included in the menu from the Quick Settings, from where you will be able to return to the old look, if you prefer.
Google also says it may take more than a few weeks to get the new UI into your personal Gmail accounts.