Various plugins and scripts for this. Here are two for Windows users. Unzip the attachment
guillotine_into_layers.exe goes in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\plug-ins
and
export-layers-plus.scm goes in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\scripts.
Find guillotine in Image -> Transform and export layers in File -> Export Layers
one minute example: https://i.imgur.com/BrHHOAI.mp4
or (and I just check this script still works) unzip sg-save-all.scm and put in your Gimp scripts folder.
Divide up your image as individual images with Image -> Slice using guides. Run Tools -> Save All.
guillotine_into_layers.exe goes in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\plug-ins
and
export-layers-plus.scm goes in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\scripts.
Find guillotine in Image -> Transform and export layers in File -> Export Layers
one minute example: https://i.imgur.com/BrHHOAI.mp4
or (and I just check this script still works) unzip sg-save-all.scm and put in your Gimp scripts folder.
Divide up your image as individual images with Image -> Slice using guides. Run Tools -> Save All.