GIMP Script-fu

sg-dissolve-group
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Command:
Dissolve Group
Menu:
"<Layers>/Dissolve Group"
PDB Name:
script-fu-sg-dissolve-group
Download:
sg-dissolve-group.scm (after the page appears, click on the "Download" command)
License:
GPLv2+

Description:

This script will "promote" the members of a layer group to its parent and remove the layer group. It is particularly useful after creating a new image by drag-n-dropping a layer group onto the Toolbox.