/etc/gimp/2.0/sessionrc is in gimp-data 2.8.10-0ubuntu1.2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 | # GIMP sessionrc
# 
# This file takes session-specific info (that is info, you want to keep
# between two GIMP sessions). You are not supposed to edit it manually, but
# of course you can do. The sessionrc will be entirely rewritten every time
# you quit GIMP. If this file isn't found, defaults are used.
(session-info "dock" "gimp-toolbox-window"
    (position 0 0)
    (size 170 820)
    (open-on-exit)
    (gimp-toolbox
        (book
            (dockable "gimp-tool-options"
                (tab-style automatic)))))
(session-info "dock" "gimp-dock-window"
    (position -0 0)
    (size 210 820)
    (open-on-exit)
    (gimp-dock
        (book
            (dockable "gimp-layer-list"
                (tab-style automatic))
            (dockable "gimp-channel-list"
                (tab-style automatic))
            (dockable "gimp-vectors-list"
                (tab-style automatic))
            (dockable "gimp-undo-history"
                (tab-style automatic)))
        (book
            (position 420)
            (dockable "gimp-brush-grid"
                (tab-style automatic))
            (dockable "gimp-pattern-grid"
                (tab-style automatic))
            (dockable "gimp-gradient-list"
                (tab-style automatic)))))
(session-info "toplevel"
    (factory-entry "gimp-empty-image-window")
    (position 410 370)
    (size 620 200))
(session-info "toplevel"
    (factory-entry "gimp-single-image-window")
    (position 160 70)
    (size 1140 780))
# end of sessionrc
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