rocknbil - 3:38 pm on Jun 18, 2012 (gmt 0)
This is not a GIMP issue, it's this way in Photoshop too. You can only save semi-transparent images as .png. That is, if a portion of the image itself is to be transparent, you can't do it with .gif (that I've ever known of.)
Gif supports transparency, but it's 100% transparent and is only useful for background knock outs or other knock outs where it won't destroy the image if the edge is sharp. There is no anti-aliasing in .gif - because it doesn't support semi-transparent pixels.
You might look at the settings in GIMP for saving under that format, but .png is mostly uncompressed, so it's a trade off. If you can figure out how to make it work with .gif, that will be the smallest file size.