checking for GTK... configure: error: Package requirements (gtk+-3.0 >= 3.2.0
                              glib-2.0 >= 2.32.0
                              gio-2.0 >= 2.26.0,
                              gmodule-2.0 >= 2.32.0
                              gthread-2.0 >= 2.32.0) were not met:

No package 'gtk+-3.0' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables GTK_CFLAGS
and GTK_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

=======> ERROR: Building '/usr/ports/opt/transmission-gtk/transmission-gtk#2.61-1.pkg.tar.gz' failed.