Gateway 2000 Inc is breaking the habit of a lifetime with its Destination home computers with the large screen and built-in television tuner, and putting them through retail outlets in the US. Up to now, the North Sioux City, South Dakota company has sold all its machines direct. The $4,000 computer will be available at Nobody Beats The Wiz stores on the East Coast this week and CompUSA Inc stores throughout the US by mid-September. It will be the only Gateway product sold through retail chains. The Pentium-based system is claimed to offer video images superior to those of a television, and both computer and television functions are controlled through a wireless keyboard and a wireless Field Mouse remote control and mouse unit. The system can be married to video cassete recorders, stereo systems and laser disk players, and can accept video feeds from cable television systems, traditional antennas, or other video devices. And audio signals from Destination can be put through home stereo systems.