It could be failing because gobject's main loop is required in order for signals to work.
The following complete example works for me:
import os
from gi.repository import Gtk, Gio
# This allows Ctrl+C to exit the program
import signal
signal.signal(signal.SIGINT, signal.SIG_DFL)
def callback(m, f, o, event):
# Without this check, multiple 'ok's will be printed for each file change
if event == Gio.FileMonitorEvent.CHANGES_DONE_HINT:
print ('ok')
gio_file = Gio.File.new_for_path(os.environ['HOME']+'/temp')
monitor = gio_file.monitor_file(Gio.FileMonitorFlags.NONE, None)
monitor.connect("changed", callback)
Gtk.main()