It's from the phrasal verb "to go on something".
From Macmillandictionary.com:
'[TRANSITIVE] [go on something] to start doing a particular activity or being in a particular state."
In your example, Robert is going to start dong a particular activity: work on a special assignment in Italy.
The words are omitted but it is understood: Robert has gone to Italy, (to work) on a special assignment . . .