Quoting Jim Brooks :

I have several image files.
These are adjacent NxN tiles.

Is there a way in GIMP to - load all of them into a grid
- modify them with GIMP
- then save them back as separate files again?

If you were to install the following two Scripts, your task might be simplified.

http://flashingtwelve.brickfilms.com/GIMP/Scripts/sg-mosaicize.scm http://flashingtwelve.brickfilms.com/GIMP/Scripts/sg-save-all-layers.scm

1. "File->New..." 2. "File->Open As Layers" -- select all of the original files, using either the CTRL key or SHIFT while selecting them. 3. Delete the original Background layer 4. If necessary, rearrange the layers in the layerstack. The bottommost layer gets placed in the upper-left of the resulting (MxN) grid and as you move up the stack, the layers should follow normal "reading" order (left to right, top to bottom). 5. Run Mosaicize ("Filters->Map->Mosaicize") 6. Delete the bottommost layer ("Frames") :
: Do all of your editing/modifications :
7. Run Save All Layers ("File->Save All Layers...") with the "Rename" checkbox unchecked (i.e., use the layernames -- which are the original filenames)

The "Save all layers" script writes the files to the directory from which GIMP was run. It is not very robust and can easily be confused by filenames with multiple periods, compression (".gz") extensions, and sometimes parentheses. It should be OK for typical filenames (I'd recommend backing up your files beforehand, though if they are not in the GIMP home directory then they should be safe).