If you want to keep the tiff pages and Gimp will open a mult-page tiff as layers then make a new-from-visible and put that layer bottom of the stack, and that is what will show in a file viewer.
Gimp will export transparent regions if all visible layers have the alpha channel activated. If you don’t want that, deactivate the alpha channel or fill the tranparent regions with white.
TIFF and PNG images support transparency (alpha channel), JPEG does not and will show white instead.