The pallete can be accessed in GIMP through the Dialog menu at the top of the picture you're editing, under Colormap. (Not where it says pallettes) However, I personally prefer to always save in GIMP's native filetype (.xcf I think) until I want to build the tileset. Then I save it as a bitmap. Finally, I use either Tileset Pal or Pallette Suite to edit the pallette.

TilesetPal is more specific, and comes with Jazz's standard pallette. It's probably better if you just want to make your pallette normal.

PalletteSuite is much better for editing pallettes, and version 3 can be used to edit pictures. However, it is more complex, and takes a bit longer if all you want to do make sure the sprites look normal.