The problem is that these are GUI apps, and crons and GNOME Schedule are designed to run command-line apps. The program has no way to attach to the display when run from these tools.
If you start the program with a script (by running "gwget &" for example) , you can capture the PID (in the special variable "$!" in bash). You can then set timers with "at" to send "STOP" and "CONT" signals with kill.
gwget ... &
# Only download at night when no one is watching...
echo "kill -s STOP $pid" | at 6AM
echo "kill -s CONT $pid" | at 10PM
It may not work cleanly for many reasons...