There are several sites dealing with Gimp, probably the most active is
http://www.gimptalk.com
Which has a variety of tutorials, however not a whole lot on working with photographs, but you can learn a variety of techniques that might be applicable to photos.
There is also
http://www.gimpguru.org/, which has a number of things relating to working with digital photographs in gimp. For plugins, go to the gimp plugin registry
http://registry.gimp.org/.
Gimp has a built in scheme interpreter, which means, if you know what you are doing, you can write some very complex add-on features, called script-fu's. Photoshop has actions, which are probably easier to use, but Gimp's method is possibly more powerful.
There is also a plugin called pspi which can allow Gimp to use some photoshop plugins. I've never used it, but it is supposedly possible to use under both windows and linux.
As for the 16 bit thing, that is correct. Rumor has it that it is in the works, but nobody knows when it will be implemented. There is a fork off of the gimp project called Cinepaint
http://www.cinepaint.org/ which supports up to 32 bit. However it is based on an older version of Gimp so doesn't have some of the newer features. It is linux only. It has been used for retouching film frames and such on movies like The Last Samurai and Harry Potter.
Also, there is UFRAW, which allows you to work in raw in linux or windows. It can run as a standalone program or as a gimp plugin. It supports 16 bit, but obviously, if you are working with Gimp, that won't be very helpful.
http://ufraw.sourceforge.net/