// On gimp 2.10
1.    Select File Menu > Open as Layers > Select all images you want to be in
    the GIF > Open
2.    In layers tab order your images > The GIF sequence will start with your
    bottom layer and run through each layer bottom to top.
3.    Select Filters from main menu > Animation > Click Optimize for GIF
4.    To view GIF > Filter > Animation > Playback
5.    To save GIF > Select File > click Export as
6.    Name your GIF and choose folder  > Select File Type > Click GIF Image
7.    Select ‘As Animation’ > Select ‘Loop Forever’.
    To change speed between each image change the delay
8.    Click Export