I hope you saved your huge xcf file containig all frames (file on which you applied "Optimize (For GIF)" filter ... step 19 in tutorial ) because your gif file is really messed up and cannot be corrected.
All you have to do is replace transparency in each of yours frames with some color (white). An easy way to do this: right click on a layer in layer dialog, "Remove Alpha Channel". Then continue with step 20.
Or... if you really want your background transparent, you can try this: in the step 22 as you save your gif file, set "save as gif" options as on the image below...

Thats all.I hope it helps. :w:

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