If you have enabled a setting to use Google’s new search interface in Chrome 29, you might see an update to Gmail as well. The main difference is that the black navigation bar with links to Google’s other web apps has been transformed into a button you need to click to bring up a list of services.
Here’s the current Gmail interface, without the “Instant Extended API” enabled:
And here’s the updated Gmail interface, with the “Instant Extended API” enabled:
To enable the update, go to chrome://flags, search “Instant Extended API” and select “Enabled” in the dropdown.