Parts of djgpp are ports of GNU sources, so those parts are GNU software. These include gcc, bison, gas, flex, etc. For these packages, you must comply with the GPL when redistributing the binaries.

However, other parts of djgpp are ports of the Berkeley BSD sources, and are public domain. This includes the C and math libraries.

Still other parts, like the extender (go32) are just plain copyrighted, but in such a way so as to allow you to use it freely.