Two Toned Scale is a scale method for GIMP.
It quite simply applies a gaussian blur of factor the same as the scale factor, the result is a smooth scale.
For best results use the cubic interpolation method, and don't use when scaling down only by small amounts.
Enter 0 in one of the width/height boxes and it will work out the width/height assuming the aspect is to be maintained.
You can install Two Toned scale by adding the .scm files in the scripts directory into one of your script directories. You can check where these are in the Folders->Scripts section of the preferences.