I think you will need to do this with a bash shell script file. If you do not understand what this means come back again and I will show you. I use a similar shell script to run radio recording scripts with mplayer or streamripper, and the only way I could get it to work was as a shell script.
Apparently the cron command line is much less able than a full bash command line, so you need to run a shell script, ie the full bash command written into a simple program and run that.
The script would be something like
in text editor, but save as backup.sh and make it executable. Then set that to run at your chosen time in gnome-schedule.
rsync -options source destination