The Gaia lock screen
By default, the lock screen is enabled, and the default PIN to unlock the device is "0000". This most likely will change over time as this feature is fleshed out.
The default Gaia interface
The default interface in Gaia is similar to what you see on most typical smartphones, as seen here.
This image is obviously of a prerelease version of the operating system, with placeholder icons (and some test applications). The status bar at the top indicates the network on which the phone is operating (or "No SIM card" for a device without a network), the network strength, WiFi signal strength, battery level, and current time.
The middle area of the display shows icons for the applications; swiping left and right pages through screens of icons.
At the bottom of the screen is a dock with room for four of your most commonly used applications. At this time, the dock can't be changed, and is preconfigured to show the Dialer, Messages, Market, and Browser apps.