Howdy
Yes, gif uses certain colour index as transparent, while png has a 8-bit channel reserved for it. This so called alpha channel provides every pixel 256 degrees of transparency. In addition gif\'s colour range is limited to 256 different colours (8-bit), while png can save 16 777 216 different colours (24-bit). Comparing Karyuu\'s avatar (gif) and the pencil (png) shows the difference. The biggest drawback with png in my opinion is the grey boxes IE users see. Also, although compressed, png\'s usually end up being larger in size than gifs, due to increased colour and transparency data.
As far as we can trust on the info on
http://www.libpng.org/, png compression is completely lossless.
Gimp is good for transparency, that I agree.

Here\'s few file formats I know save transparency.
Image formats that support alpha channel(s):
-Portable Network Graphic (.png)
-Multiple-image Network Graphics (.mng)
-JPEG Network Graphics (.jng)
-Truevision TarGA image (.tga)
-Tagged Image File Format (.tif; .tiff)
-Paint Shop Pro image (.psp)
-Adobe Photoshop image (.psd)
-Gimp XCF image (.xcf)
-Corel Photopaint (.cpt)
Image formats that support keycolor transparency:
-Portable Network Graphic (.png)
-Graphics Interchange Format (.gif)
-MS Windows icon (.ico)
-X PixMap image (.xpm)
My signature images are png too.