Try turning off the mirror setting, and instead use the FN+F7 (or Function Key that enables the video output thru the 15 pin connection. I know on Dell computers, it's FN+F7, and on ThinkPads it may be FN+F4. Each manufacturer is different. But I hook my laptop up to an external monitor every day and have to go through the process when starting the computer.
Also to keep in mind, you may try the Fn+F# combination once and get the same error. That means it's going to mirror. What you want to do is have the computer send the signal ONLY to the television, so that may require hitting the FN+F# key a second time. At that point, it will send it only to the television, and that should do the trick. I don't know what Gateway's F-key funtion key for external monitor enabling is, but if you can figure that out (usually a picture of a box or it may say LCD), that would be my suggestion.