The GIF coder does not use the Quality setting and the CompressionMethod will always be CompressionMethod.LZW. You should do the following if you want to optimize the output file:
Make sure you upgrade to the latest version, the Optimize/OptimizeTransparency methods were bugged in previous versions.
using (MagickImageCollection col = new MagickImageCollection(@"C:/PathToGif")) { col.Coalesce(); AddOtherImages(col); col.Optimize(); col.OptimizeTransparency(); col.Write(@"C:/Path/To/Output"); }