In most cases, there aren't any.

If you don't use libgpp.a (C++ class library) or libgpl.a (obstack, regexp and other GNU packages), don't use bison, and don't rebuild go32.exe, then you may do whatever you like with your programs. You may redistribute the binaries of go32, emu387, and the graphics drivers as they are shipped with djgpp as long as you tell your customers how to get djgpp.

The GNU GPL/LGPL (General Public License/Library General Public License) will apply if you link libgpp.a or libgpl.a and you do not use gcc to compile your program, or use bison, change go32's sources, or use any other FSF sources which fall under GPL (like gcc's sources) as part of your program (e.g., linking your application with GNU getopt or regex packages which come with many GNU programs).

Note that libgpp.a may be used without incurring the GPL if your application was itself compiled with gcc, in which case the library allows you to use it binary-wise without restriction.