Change colour of a particular area of GIF

GIMP 3.0 RC1

I tried changing the colour of a particular area of GIF by colour selecting it. It was a money bag GIF where coin are raining (sort of). I tried to change the colour of money bag by selecting both the shadow colour and bag colour. Luckily 3.0 allows selecting layers, but when I got to colours and try to apply Hue, it is not available for multiple layers.

Then I used layer effect and was able to change the colour of the area I wanted to. But there is no option to export it. I tried it says cannot export as GIF in the last step. Then I tried to apply the layer effects on Layer Group (which is 2 Hue) and select the layers to export. I did all these in Windows 10 and Ubuntu 24.04. But no option to merge down the layer effects. Which mean, I am able to change the color but not export.

Is there any way around or other method available to achieve this? I tried to change the image from RGB to Indexed. But it is greyed out. Then I changed to Indexed first, now I cannot import my GIF at all. I read that unoptimising GIF helps. Even that failed. Any help would be great. Thanks.