You have to path to the executable or include PATH in the crontab itself.
so if you ran command -v grsync it would give you the full path, you’d use this full path instead of just grsync. Or you could just stick the path at the top of the crontab file. echo $PATH and then take the output and put it into line 1 of the crontab as a variable like PATH="put the path output here"
Read through this, they may make better sense than me. Plus I’m tired. http://stackoverflow.com/questions/2388087/how-to-get-cron-to-call-in-the-correct-paths