It's the one being shown on the bottom right.
Keeps coming when I visit Gmail after closing the popup. Is it possible to disable it permanently?
Someone has created a userscript to remove the notification bar.
According to a discussion on Google Groups the behaviour of the popup is intended. The popup will be removed 14 days after you have started using the new look.
We wanted that link to be available to you for the first couple of weeks that they were using the new look so that you could easily access some info about the changes and send us feedback. You can dismiss it per session by hitting the "X" (but it will come back on the next reload, and we will remove it completely 14 days after you first started using the new look.