Gig-E is a communication standard for a wide range of cameras. OpenCV now contains a wrapper for The Prosilica Gig-E based cameras (see CV_CAP_PVAPI)
But in general it’s better to use the camera’s native API to get the data and then use openCV to convert the returned data into an image, openCv contains a number of Bayer pattern ->RGB routines.
The CvCapture module is convenient for testing, because it can seemlessly read from a camera or a file – but it’s not really suitable for high-speed real-time vision