I develop gFTP on a Debian GNU/Linux machine. It should run under any other Linux distributions. Here is a list of other platforms I have received successful compile reports:
FreeBSD/NetBSD/OpenBSD/BSDI
Solaris
IRIX
Digital UNIX
HP/UX 11
If gFTP compiles and runs on a platform not listed here, please email me about it and I'll add it to this list. If you are having problems compiling GTK+ or gFTP on another platform, try using GNU make instead of your vendor supplied make.