No info about Gimp but keep in mind TGAs don't support 32 bit unless they've got an alpha layer. A TGA can only be 24 bit without the alpha. Maybe adding an alpha in Gimp will make it 32 bit?
Hey Stormtrooper, try using the menu option "Layer : Transparency : Add Alpha Layer". That seemed to generate a blank alpha layer and successfully saved it in the .TGA format.