All Layers Offset is a GIMP Python plug-in. It's forked from Group Offset by Vesa Kivimäki (http://registry.gimp.org/node/26996) This script allows you to apply an offset to all your layers.
If you don't know how to install a GIMP plug-in, see https://en.wikibooks.org/wiki/GIMP/Installing_Plugins
Once installed, you can execute the script via GIMP -> Group -> All Layers Offset...