パレットマップ フィルターは パレットダイアログ で選ばれた現在のパレットの色を利用して画像の色を差し替えます。

It maps the contents of the specified drawable (layer, selection...) with the active palette. It calculates luminosity of each pixel and replaces the pixel by the palette sample at the corresponding index. A complete black pixel becomes the lowest palette entry, and complete white becomes the highest. Works on both Grayscale and RGB images with or without alpha channel.

This filter is found in the image window menu under Colors → Map → Palette Map.