1) convert the RGB to Index color
2) flatten the image
3) save it as gif
4) close
Would do the job. I was wrong. The Alpha Channel cause Photoshop keep asking me, if I want to save the changed image for each of these files. Holding the "N" key helped to straighten things up, however I do wonder if for the next time, there is some clean way how to get rid of the Alpha Channel or surpress the warning when closing the file, because I was not be able to find a function, that let me do it.
That's right. Suggestions like "Preferences / General / Reset all warning dialogs" does not change a thing

Anyone know how to make Photoshop warnings about closing changed image go away?