Which Gimp version? What settings? On my Gimp 2.10.14, with no compression, a 400x400 image with a single layer without alpha channel exports as a 664K file, and reloads in Gimp without getting an alpha-channel. Adding an alpha-channel makes it a 820K file.
Edit: this is confirmed by ImageMagick's identify that says: tiff:alpha: unspecified for the image without alpha and tiff:alpha: unassociated for the image with alpha.
PS: Can you attach a (small) TIFF from an image without alpha channel in Gimp?
Edit: this is confirmed by ImageMagick's identify that says: tiff:alpha: unspecified for the image without alpha and tiff:alpha: unassociated for the image with alpha.
PS: Can you attach a (small) TIFF from an image without alpha channel in Gimp?