I have never used GIMP but I have photoshop CS5 and there is an option for quality setting when I resize an image. I have resized images without losing quality by choosing a high quality setting. Hopefully someone can tell you exactly how to do it with GIMP because all I can say is to look for that option or something similar.
**As I was typing this response I thought to do a google search for "resize image" and this is what came up. Never tried any of them but it can hurt. Link is below.
resize image - Google Search