Hi quoc,
Yes, you will actually get coordinates passed the edges of the screen right out of the box using the EyeX SDK.
In the gaze point data stream, the gaze point coordinates are given in pixels on the virtual screen. Your (primary) monitor will have coordinate (0,0) at its upper left corner.
On a (primary) monitor with resolution 1920 x 1080, you can get gaze points like (-50, 100), (100, -50), (1970, 100), (100, 1130) which correspond to 50 pixels to the left/above/to the right/below the visible screen respectively.
You can see how the coordinates work by running one of the MinimalGazeDataStream samples (in EyeX SDK for .NET or C/C++). These samples write the stream of gaze points to a console window.
The screen size in pixels and millimeters is available through the EyeX API as well, so you can translate the pixel values to millimeters if you need to.