Hmm...i have to fix some things here cause we worked together, Jon and I, developing 2 tools in parallel.
His as the advantage to let you edit files with weaks you listed plus one : it only displays team's gpi files.
Mine can't edit files but can preview every gip files as acdsee does for any picture format.
Untill a recent time, i wasn't able to export gpi to tga but i hope to do this in a next future.
About gpi sizes, you're wrong, Jon and I lost many time searching how to find them but in fact they are all listed in a file near the gpi folder, so every gpi file as it's own size declared and gp4 use that size to read gpi file.
So gpi will be editable one day.