You should only scale bitmap images down- never up. Scaling pixels up forces the program to guess which pixels are valid data. I blame TV shows like CSI for their investigation shots at the police station. You can almost hear it now, officers crowded around a monitor with a pixelated photo from a robbery the night before. "Enhance!" they command and the image zooms in fluidly despite the fact that there wasn't any more pixel data to be shown.
Your best bet is to recreate the gradient entirely at the correct size from the start using a free program such as GIMP.