If you use gtkd-devel
package provided by Fedora, you should know that it is linked against libphobos-ldc
library which you will get once you install ldc2
package (yum install ldc2
will do).
This does not yet solve your problem though... gtkd-devel package installs gtkd D interface modules into the /usr/include/d/gtkd-2
folder. So, in order to compile your GtkD application you need to execute something like (assuming your D file has name "firstwin.d"): ldc2 -I/usr/include/d/gtkd-2 -L-lgtkd-2 firstwin.d