aseprite usually adds transparent colour when it opens gif file, if you don’t have transparency in your sprite, you can delete if it bothers you (you have to be in rgb mode for that).
otherwise aseprite stores to colour table all the colours which have been used in gif. you can get rid of all colours which are not directly part of an image by switching to rgb mode and creating new palette from sprite: How to create a palette with the colors of the current sprite?
if the palette still has more colours than expected, it means there are used somewhere in the image. in that case you want to use replace colour function. go to edit > replace color.
https://techstacker.com/replace-a-color-everywhere-aseprite/
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