I suggest not to use GIMP. But you might want to anyways.
I believe that graphicsgale is the best for editing the GFX files, it is simple and good ;)
And to replace the use Reshacker.
OR
For those two in one use "XN Resource Editor" it's alright.
but this is me ;)
BTW, to edit GFX files you need Reshacker and save or copy files one by one or take the whole thing in 1 go.
They will come up as 1 frame at a time.