"Gruger" something is a term that I often came across in my school-boy life - I always was under the impression that it was very informal, so I'm surprised to see it in such a context.
Wiki renders the meaning nicely:
http://en.wiktionary.org/wiki/grugerBut it can be used in many different ways:
A couple of example sentences:
"Il m'a grugé dans la queue" => "he pushed in front of me in the queue"
"Il m'a grugé un sandwich" => "he nicked my sandwich [when I wasn't looking]"
So basically, it means to
con somebody out of somethingIn this context therefore, I think "time wasting" works very well, and that is probably the translation that I would use.